))
consisting of the symbol `cond' followed by parenthesized pairs of
expressions
( )
called "clauses". The first expression in each pair is a "predicate"that
is, an expression whose value is interpreted as either true or false.(1)
Conditional expressions are evaluated as follows. The predicate
is evaluated first. If its value is false, then is
evaluated. If 's value is also false, then is evaluated.
This process continues until a predicate is found whose value is true,
in which case the interpreter returns the value of the corresponding expression
"consequent expression" of the clause as the value of the
conditional expression. If none of the
's is found to be true, the
value of the `cond' is undefined.
The word "predicate" is used for procedures that return true or
false, as well as for expressions that evaluate to true or false. The
absolutevalue procedure `abs' makes use of the primitive predicates
`>', `<', and `='.(2) These take two numbers as arguments and test
whether the first number is, respectively, greater than, less than, or
equal to the second number, returning true or false accordingly.
Another way to write the absolutevalue procedure is
(define (abs x)
(cond ((< x 0) ( x))
(else x)))
which could be expressed in English as "If x is less than zero return 
x; otherwise return x." `Else' is a special symbol that can be used in
place of the
in the final clause of a `cond'. This causes the
`cond' to return as its value the value of the corresponding
whenever all previous clauses have been bypassed. In fact, any
expression that always evaluates to a true value could be used as the
here.
Here is yet another way to write the absolutevalue procedure:
(define (abs x)
(if (< x 0)
( x)
x))
This uses the special form `if', a restricted type of conditional
that can be used when there are precisely two cases in the case
analysis. The general form of an `if' expression is
(if )
To evaluate an `if' expression, the interpreter starts by evaluating
the part of the expression. If the evaluates
to a true value, the interpreter then evaluates the and
returns its value. Otherwise it evaluates the and returns
its value.(3)
In addition to primitive predicates such as `<', `=', and `>', there
are logical composition operations, which enable us to construct
compound predicates. The three most frequently used are these:
* `(and ... )'
The interpreter evaluates the expressions one at a time, in
lefttoright order. If any evaluates to false, the value of
the `and' expression is false, and the rest of the 's are not
evaluated. If all 's evaluate to true values, the value of the
`and' expression is the value of the last one.
* `(or ... )'
The interpreter evaluates the expressions one at a time, in
lefttoright order. If any evaluates to a true value, that
value is returned as the value of the `or' expression, and the
rest of the 's are not evaluated. If all 's evaluate to
false, the value of the `or' expression is false.
* `(not )'
The value of a `not' expression is true when the expression
evaluates to false, and false otherwise.
Notice that `and' and `or' are special forms, not procedures, because
the subexpressions are not necessarily all evaluated. `Not' is an
ordinary procedure.
As an example of how these are used, the condition that a number x
be in the range 5 < x < 10 may be expressed as
(and (> x 5) (< x 10))
As another example, we can define a predicate to test whether one
number is greater than or equal to another as
(define (>= x y)
(or (> x y) (= x y)))
or alternatively as
(define (>= x y)
(not (< x y)))
*Exercise 1.1:* Below is a sequence of expressions. What is the
result printed by the interpreter in response to each expression?
Assume that the sequence is to be evaluated in the order in which
it is presented.
10
(+ 5 3 4)
( 9 1)
(/ 6 2)
(+ (* 2 4) ( 4 6))
(define a 3)
(define b (+ a 1))
(+ a b (* a b))
(= a b)
(if (and (> b a) (< b (* a b)))
b
a)
(cond ((= a 4) 6)
((= b 4) (+ 6 7 a))
(else 25))
(+ 2 (if (> b a) b a))
(* (cond ((> a b) a)
((< a b) b)
(else 1))
(+ a 1))
*Exercise 1.2:* Translate the following expression into prefix
form.
5 + 4 + (2  (3  (6 + 4/5)))

3(6  2)(2  7)
*Exercise 1.3:* Define a procedure that takes three numbers as
arguments and returns the sum of the squares of the two larger
numbers.
*Exercise 1.4:* Observe that our model of evaluation allows for
combinations whose operators are compound expressions. Use this
observation to describe the behavior of the following procedure:
(define (aplusabsb a b)
((if (> b 0) + ) a b))
*Exercise 1.5:* Ben Bitdiddle has invented a test to determine
whether the interpreter he is faced with is using
applicativeorder evaluation or normalorder evaluation. He
defines the following two procedures:
(define (p) (p))
(define (test x y)
(if (= x 0)
0
y))
Then he evaluates the expression
(test 0 (p))
What behavior will Ben observe with an interpreter that uses
applicativeorder evaluation? What behavior will he observe with
an interpreter that uses normalorder evaluation? Explain your
answer. (Assume that the evaluation rule for the special form
`if' is the same whether the interpreter is using normal or
applicative order: The predicate expression is evaluated first,
and the result determines whether to evaluate the consequent or
the alternative expression.)
 Footnotes 
(1) "Interpreted as either true or false" means this: In Scheme,
there are two distinguished values that are denoted by the constants
`#t' and `#f'. When the interpreter checks a predicate's value, it
interprets `#f' as false. Any other value is treated as true. (Thus,
providing `#t' is logically unnecessary, but it is convenient.) In
this book we will use names `true' and `false', which are associated
with the values `#t' and `#f' respectively.
(2) `Abs' also uses the "minus" operator `', which, when used with
a single operand, as in `( x)', indicates negation.
(3) A minor difference between `if' and `cond' is that the part
of each `cond' clause may be a sequence of expressions. If the
corresponding
is found to be true, the expressions are
evaluated in sequence and the value of the final expression in the
sequence is returned as the value of the `cond'. In an `if'
expression, however, the and must be single
expressions.
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1.1.7 Example: Square Roots by Newton's Method

Procedures, as introduced above, are much like ordinary mathematical
functions. They specify a value that is determined by one or more
parameters. But there is an important difference between mathematical
functions and computer procedures. Procedures must be effective.
As a case in point, consider the problem of computing square roots.
We can define the squareroot function as
sqrt(x) = the y such that y >= 0 and y^2 = x
This describes a perfectly legitimate mathematical function. We
could use it to recognize whether one number is the square root of
another, or to derive facts about square roots in general. On the
other hand, the definition does not describe a procedure. Indeed, it
tells us almost nothing about how to actually find the square root of a
given number. It will not help matters to rephrase this definition in
pseudoLisp:
(define (sqrt x)
(the y (and (>= y 0)
(= (square y) x))))
This only begs the question.
The contrast between function and procedure is a reflection of the
general distinction between describing properties of things and
describing how to do things, or, as it is sometimes referred to, the
distinction between declarative knowledge and imperative knowledge. In
mathematics we are usually concerned with declarative (what is)
descriptions, whereas in computer science we are usually concerned with
imperative (how to) descriptions.(1)
How does one compute square roots? The most common way is to use
Newton's method of successive approximations, which says that whenever
we have a guess y for the value of the square root of a number x, we
can perform a simple manipulation to get a better guess (one closer to
the actual square root) by averaging y with x/y.(2) For example, we can
compute the square root of 2 as follows. Suppose our initial guess is
1:
Guess Quotient Average
1 (2/1) = 2 ((2 + 1)/2) = 1.5
1.5 (2/1.5) = 1.3333 ((1.3333 + 1.5)/2) = 1.4167
1.4167 (2/1.4167) = 1.4118 ((1.4167 + 1.4118)/2) = 1.4142
1.4142 ... ...
Continuing this process, we obtain better and better approximations to
the square root.
Now let's formalize the process in terms of procedures. We start
with a value for the radicand (the number whose square root we are
trying to compute) and a value for the guess. If the guess is good
enough for our purposes, we are done; if not, we must repeat the
process with an improved guess. We write this basic strategy as a
procedure:
(define (sqrtiter guess x)
(if (goodenough? guess x)
guess
(sqrtiter (improve guess x)
x)))
A guess is improved by averaging it with the quotient of the
radicand and the old guess:
(define (improve guess x)
(average guess (/ x guess)))
where
(define (average x y)
(/ (+ x y) 2))
We also have to say what we mean by "good enough." The following
will do for illustration, but it is not really a very good test. (See
exercise *Note Exercise 17::.) The idea is to improve the answer
until it is close enough so that its square differs from the radicand
by less than a predetermined tolerance (here 0.001):(3)
(define (goodenough? guess x)
(< (abs ( (square guess) x)) 0.001))
Finally, we need a way to get started. For instance, we can always
guess that the square root of any number is 1:(4)
(define (sqrt x)
(sqrtiter 1.0 x))
If we type these definitions to the interpreter, we can use `sqrt'
just as we can use any procedure:
(sqrt 9)
3.00009155413138
(sqrt (+ 100 37))
11.704699917758145
(sqrt (+ (sqrt 2) (sqrt 3)))
1.7739279023207892
(square (sqrt 1000))
1000.000369924366
The `sqrt' program also illustrates that the simple procedural
language we have introduced so far is sufficient for writing any purely
numerical program that one could write in, say, C or Pascal. This
might seem surprising, since we have not included in our language any
iterative (looping) constructs that direct the computer to do something
over and over again. `Sqrtiter', on the other hand, demonstrates how
iteration can be accomplished using no special construct other than the
ordinary ability to call a procedure.(5)
*Exercise 1.6:* Alyssa P. Hacker doesn't see why `if' needs to be
provided as a special form. "Why can't I just define it as an
ordinary procedure in terms of `cond'?" she asks. Alyssa's friend
Eva Lu Ator claims this can indeed be done, and she defines a new
version of `if':
(define (newif predicate thenclause elseclause)
(cond (predicate thenclause)
(else elseclause)))
Eva demonstrates the program for Alyssa:
(newif (= 2 3) 0 5)
5
(newif (= 1 1) 0 5)
0
Delighted, Alyssa uses `newif' to rewrite the squareroot program:
(define (sqrtiter guess x)
(newif (goodenough? guess x)
guess
(sqrtiter (improve guess x)
x)))
What happens when Alyssa attempts to use this to compute square
roots? Explain.
*Exercise 1.7:* The `goodenough?' test used in computing square
roots will not be very effective for finding the square roots of
very small numbers. Also, in real computers, arithmetic operations
are almost always performed with limited precision. This makes
our test inadequate for very large numbers. Explain these
statements, with examples showing how the test fails for small and
large numbers. An alternative strategy for implementing
`goodenough?' is to watch how `guess' changes from one iteration
to the next and to stop when the change is a very small fraction
of the guess. Design a squareroot procedure that uses this kind
of end test. Does this work better for small and large numbers?
*Exercise 1.8:* Newton's method for cube roots is based on the
fact that if y is an approximation to the cube root of x, then a
better approximation is given by the value
x/y^2 + 2y

3
Use this formula to implement a cuberoot procedure analogous to
the squareroot procedure. (In section *Note 134:: we will see
how to implement Newton's method in general as an abstraction of
these squareroot and cuberoot procedures.)
 Footnotes 
(1) Declarative and imperative descriptions are intimately related,
as indeed are mathematics and computer science. For instance, to say
that the answer produced by a program is "correct" is to make a
declarative statement about the program. There is a large amount of
research aimed at establishing techniques for proving that programs are
correct, and much of the technical difficulty of this subject has to do
with negotiating the transition between imperative statements (from
which programs are constructed) and declarative statements (which can be
used to deduce things). In a related vein, an important current area in
programminglanguage design is the exploration of socalled very
highlevel languages, in which one actually programs in terms of
declarative statements. The idea is to make interpreters sophisticated
enough so that, given "what is" knowledge specified by the programmer,
they can generate "how to" knowledge automatically. This cannot be
done in general, but there are important areas where progress has been
made. We shall revisit this idea in *Note Chapter 4::.
(2) This squareroot algorithm is actually a special case of
Newton's method, which is a general technique for finding roots of
equations. The squareroot algorithm itself was developed by Heron of
Alexandria in the first century A.D. We will see how to express the
general Newton's method as a Lisp procedure in section *Note 134::.
(3) We will usually give predicates names ending with question
marks, to help us remember that they are predicates. This is just a
stylistic convention. As far as the interpreter is concerned, the
question mark is just an ordinary character.
(4) Observe that we express our initial guess as 1.0 rather than 1.
This would not make any difference in many Lisp implementations. MIT
Scheme, however, distinguishes between exact integers and decimal
values, and dividing two integers produces a rational number rather
than a decimal. For example, dividing 10 by 6 yields 5/3, while
dividing 10.0 by 6.0 yields 1.6666666666666667. (We will learn how to
implement arithmetic on rational numbers in section *Note 211::.) If
we start with an initial guess of 1 in our squareroot program, and x
is an exact integer, all subsequent values produced in the squareroot
computation will be rational numbers rather than decimals. Mixed
operations on rational numbers and decimals always yield decimals, so
starting with an initial guess of 1.0 forces all subsequent values to
be decimals.
(5) Readers who are worried about the efficiency issues involved in
using procedure calls to implement iteration should note the remarks on
"tail recursion" in section *Note 121::.
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1.1.8 Procedures as BlackBox Abstractions

`Sqrt' is our first example of a process defined by a set of mutually
defined procedures. Notice that the definition of `sqrtiter' is "recursive";
that is, the procedure is defined in terms of itself. The idea of
being able to define a procedure in terms of itself may be disturbing;
it may seem unclear how such a "circular" definition could make sense
at all, much less specify a welldefined process to be carried out by a
computer. This will be addressed more carefully in section *Note
12::. But first let's consider some other important points
illustrated by the `sqrt' example.
Observe that the problem of computing square roots breaks up
naturally into a number of subproblems: how to tell whether a guess is
good enough, how to improve a guess, and so on. Each of these tasks is
accomplished by a separate procedure. The entire `sqrt' program can be
viewed as a cluster of procedures (shown in *Note Figure 12::) that
mirrors the decomposition of the problem into subproblems.
*Figure 1.2:* Procedural decomposition of the `sqrt' program.
sqrt

sqrtiter
/ \
goodenough improve
/ \ 
square abs average
The importance of this decomposition strategy is not simply that one
is dividing the program into parts. After all, we could take any large
program and divide it into partsthe first ten lines, the next ten
lines, the next ten lines, and so on. Rather, it is crucial that each
procedure accomplishes an identifiable task that can be used as a
module in defining other procedures. For example, when we define the
`goodenough?' procedure in terms of `square', we are able to regard
the `square' procedure as a "black box." We are not at that moment
concerned with _how_ the procedure computes its result, only with the
fact that it computes the square. The details of how the square is
computed can be suppressed, to be considered at a later time. Indeed,
as far as the `goodenough?' procedure is concerned, `square' is not
quite a procedure but rather an abstraction of a procedure, a socalled "procedural
abstraction". At this level of abstraction, any procedure that
computes the square is equally good.
Thus, considering only the values they return, the following two
procedures for squaring a number should be indistinguishable. Each
takes a numerical argument and produces the square of that number as
the value.(1)
(define (square x) (* x x))
(define (square x)
(exp (double (log x))))
(define (double x) (+ x x))
So a procedure definition should be able to suppress detail. The
users of the procedure may not have written the procedure themselves,
but may have obtained it from another programmer as a black box. A
user should not need to know how the procedure is implemented in order
to use it.
Local names
...........
One detail of a procedure's implementation that should not matter to
the user of the procedure is the implementer's choice of names for the
procedure's formal parameters. Thus, the following procedures should
not be distinguishable:
(define (square x) (* x x))
(define (square y) (* y y))
This principlethat the meaning of a procedure should be
independent of the parameter names used by its authorseems on the
surface to be selfevident, but its consequences are profound. The
simplest consequence is that the parameter names of a procedure must be
local to the body of the procedure. For example, we used `square' in
the definition of `goodenough?' in our squareroot procedure:
(define (goodenough? guess x)
(< (abs ( (square guess) x)) 0.001))
The intention of the author of `goodenough?' is to determine if the
square of the first argument is within a given tolerance of the second
argument. We see that the author of `goodenough?' used the name
`guess' to refer to the first argument and `x' to refer to the second
argument. The argument of `square' is `guess'. If the author of
`square' used `x' (as above) to refer to that argument, we see that the
`x' in `goodenough?' must be a different `x' than the one in `square'.
Running the procedure `square' must not affect the value of `x' that
is used by `goodenough?', because that value of `x' may be needed by
`goodenough?' after `square' is done computing.
If the parameters were not local to the bodies of their respective
procedures, then the parameter `x' in `square' could be confused with
the parameter `x' in `goodenough?', and the behavior of `goodenough?'
would depend upon which version of `square' we used. Thus, `square'
would not be the black box we desired.
A formal parameter of a procedure has a very special role in the
procedure definition, in that it doesn't matter what name the formal
parameter has. Such a name is called a "bound variable", and we say
that the procedure definition "binds" its formal parameters. The
meaning of a procedure definition is unchanged if a bound variable is
consistently renamed throughout the definition.(2) If a variable is
not bound, we say that it is "free". The set of expressions for which
a binding defines a name is called the "scope" of that name. In a
procedure definition, the bound variables declared as the formal
parameters of the procedure have the body of the procedure as their
scope.
In the definition of `goodenough?' above, `guess' and `x' are bound
variables but `<', `', `abs', and `square' are free. The meaning of
`goodenough?' should be independent of the names we choose for `guess'
and `x' so long as they are distinct and different from `<', `',
`abs', and `square'. (If we renamed `guess' to `abs' we would have
introduced a bug by "capturing" the variable `abs'. It would have
changed from free to bound.) The meaning of `goodenough?' is not
independent of the names of its free variables, however. It surely
depends upon the fact (external to this definition) that the symbol
`abs' names a procedure for computing the absolute value of a number.
`Goodenough?' will compute a different function if we substitute `cos'
for `abs' in its definition.
Internal definitions and block structure
........................................
We have one kind of name isolation available to us so far: The formal
parameters of a procedure are local to the body of the procedure. The
squareroot program illustrates another way in which we would like to
control the use of names. The existing program consists of separate
procedures:
(define (sqrt x)
(sqrtiter 1.0 x))
(define (sqrtiter guess x)
(if (goodenough? guess x)
guess
(sqrtiter (improve guess x) x)))
(define (goodenough? guess x)
(< (abs ( (square guess) x)) 0.001))
(define (improve guess x)
(average guess (/ x guess)))
The problem with this program is that the only procedure that is
important to users of `sqrt' is `sqrt'. The other procedures
(`sqrtiter', `goodenough?', and `improve') only clutter up their
minds. They may not define any other procedure called `goodenough?'
as part of another program to work together with the squareroot
program, because `sqrt' needs it. The problem is especially severe in
the construction of large systems by many separate programmers. For
example, in the construction of a large library of numerical
procedures, many numerical functions are computed as successive
approximations and thus might have procedures named `goodenough?' and
`improve' as auxiliary procedures. We would like to localize the
subprocedures, hiding them inside `sqrt' so that `sqrt' could coexist
with other successive approximations, each having its own private
`goodenough?' procedure. To make this possible, we allow a procedure
to have internal definitions that are local to that procedure. For
example, in the squareroot problem we can write
(define (sqrt x)
(define (goodenough? guess x)
(< (abs ( (square guess) x)) 0.001))
(define (improve guess x)
(average guess (/ x guess)))
(define (sqrtiter guess x)
(if (goodenough? guess x)
guess
(sqrtiter (improve guess x) x)))
(sqrtiter 1.0 x))
Such nesting of definitions, called "block structure", is basically
the right solution to the simplest namepackaging problem. But there
is a better idea lurking here. In addition to internalizing the
definitions of the auxiliary procedures, we can simplify them. Since
`x' is bound in the definition of `sqrt', the procedures
`goodenough?', `improve', and `sqrtiter', which are defined
internally to `sqrt', are in the scope of `x'. Thus, it is not
necessary to pass `x' explicitly to each of these procedures. Instead,
we allow `x' to be a free variable in the internal definitions, as
shown below. Then `x' gets its value from the argument with which the
enclosing procedure `sqrt' is called. This discipline is called "lexical
scoping".(3)
(define (sqrt x)
(define (goodenough? guess)
(< (abs ( (square guess) x)) 0.001))
(define (improve guess)
(average guess (/ x guess)))
(define (sqrtiter guess)
(if (goodenough? guess)
guess
(sqrtiter (improve guess))))
(sqrtiter 1.0))
We will use block structure extensively to help us break up large
programs into tractable pieces.(4) The idea of block structure
originated with the programming language Algol 60. It appears in most
advanced programming languages and is an important tool for helping to
organize the construction of large programs.
 Footnotes 
(1) It is not even clear which of these procedures is a more
efficient implementation. This depends upon the hardware available.
There are machines for which the "obvious" implementation is the less
efficient one. Consider a machine that has extensive tables of
logarithms and antilogarithms stored in a very efficient manner.
(2) The concept of consistent renaming is actually subtle and
difficult to define formally. Famous logicians have made embarrassing
errors here.
(3) [Footnote 28] Lexical scoping dictates that free variables in a
procedure are taken to refer to bindings made by enclosing procedure
definitions; that is, they are looked up in the environment in which the
procedure was defined. We will see how this works in detail in chapter
3 when we study environments and the detailed behavior of the
interpreter.
(4) Embedded definitions must come first in a procedure body. The
management is not responsible for the consequences of running programs
that intertwine definition and use.
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1.2 Procedures and the Processes They Generate
==============================================
We have now considered the elements of programming: We have used
primitive arithmetic operations, we have combined these operations, and
we have abstracted these composite operations by defining them as
compound procedures. But that is not enough to enable us to say that
we know how to program. Our situation is analogous to that of someone
who has learned the rules for how the pieces move in chess but knows
nothing of typical openings, tactics, or strategy. Like the novice
chess player, we don't yet know the common patterns of usage in the
domain. We lack the knowledge of which moves are worth making (which
procedures are worth defining). We lack the experience to predict the
consequences of making a move (executing a procedure).
The ability to visualize the consequences of the actions under
consideration is crucial to becoming an expert programmer, just as it
is in any synthetic, creative activity. In becoming an expert
photographer, for example, one must learn how to look at a scene and
know how dark each region will appear on a print for each possible
choice of exposure and development conditions. Only then can one
reason backward, planning framing, lighting, exposure, and development
to obtain the desired effects. So it is with programming, where we are
planning the course of action to be taken by a process and where we
control the process by means of a program. To become experts, we must
learn to visualize the processes generated by various types of
procedures. Only after we have developed such a skill can we learn to
reliably construct programs that exhibit the desired behavior.
A procedure is a pattern for the "local evolution" of a computational
process. It specifies how each stage of the process is built upon the
previous stage. We would like to be able to make statements about the
overall, or "global", behavior of a process whose local evolution has
been specified by a procedure. This is very difficult to do in
general, but we can at least try to describe some typical patterns of
process evolution.
In this section we will examine some common "shapes" for processes
generated by simple procedures. We will also investigate the rates at
which these processes consume the important computational resources of
time and space. The procedures we will consider are very simple.
Their role is like that played by test patterns in photography: as
oversimplified prototypical patterns, rather than practical examples in
their own right.
* Menu:
* 121:: Linear Recursion and Iteration
* 122:: Tree Recursion
* 123:: Orders of Growth
* 124:: Exponentiation
* 125:: Greatest Common Divisors
* 126:: Example: Testing for Primality
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1.2.1 Linear Recursion and Iteration

*Figure 1.3:* A linear recursive process for computing 6!.
(factorial 6) .
(* 6 (factorial 5)) 
(* 6 (* 5 (factorial 4))) 
(* 6 (* 5 (* 4 (factorial 3)))) 
(* 6 (* 5 (* 4 (* 3 (factorial 2))))) 
(* 6 (* 5 (* 4 (* 3 (* 2 (factorial 1)))))) 
(* 6 (* 5 (* 4 (* 3 (* 2 1))))) 
(* 6 (* 5 (* 4 (* 3 2)))) 
(* 6 (* 5 (* 4 6))) 
(* 6 (* 5 24)) 
(* 6 120) 
720 <'
We begin by considering the factorial function, defined by
n! = n * (n  1) * (n  2) ... 3 * 2 * 1
There are many ways to compute factorials. One way is to make use
of the observation that n! is equal to n times (n  1)! for any positive
integer n:
n! = n * [(n  1) * (n  2) ... 3 * 2 * 1] = n * (n  1)!
Thus, we can compute n! by computing (n  1)! and multiplying the
result by n. If we add the stipulation that 1! is equal to 1, this
observation translates directly into a procedure:
(define (factorial n)
(if (= n 1)
1
(* n (factorial ( n 1)))))
We can use the substitution model of section *Note 115:: to watch
this procedure in action computing 6!, as shown in *Note Figure 13::.
Now let's take a different perspective on computing factorials. We
could describe a rule for computing n! by specifying that we first
multiply 1 by 2, then multiply the result by 3, then by 4, and so on
until we reach n. More formally, we maintain a running product,
together with a counter that counts from 1 up to n. We can describe
the computation by saying that the counter and the product
simultaneously change from one step to the next according to the rule
product < counter ... product
counter < counter + 1
and stipulating that n! is the value of the product when the counter
exceeds n.
*Figure 1.4:* A linear iterative process for computing 6!.
(factorial 6) .
(factiter 1 1 6) 
(factiter 1 2 6) 
(factiter 2 3 6) 
(factiter 6 4 6) 
(factiter 24 5 6) 
(factiter 120 6 6) 
(factiter 720 7 6) V
720
Once again, we can recast our description as a procedure for
computing factorials:(1)
(define (factorial n)
(factiter 1 1 n))
(define (factiter product counter maxcount)
(if (> counter maxcount)
product
(factiter (* counter product)
(+ counter 1)
maxcount)))
As before, we can use the substitution model to visualize the
process of computing 6!, as shown in *Note Figure 14::.
Compare the two processes. From one point of view, they seem hardly
different at all. Both compute the same mathematical function on the
same domain, and each requires a number of steps proportional to n to
compute n!. Indeed, both processes even carry out the same sequence of
multiplications, obtaining the same sequence of partial products. On
the other hand, when we consider the "shapes" of the two processes, we
find that they evolve quite differently.
Consider the first process. The substitution model reveals a shape
of expansion followed by contraction, indicated by the arrow in *Note
Figure 13::. The expansion occurs as the process builds up a chain of operations
"deferred operations" (in this case, a chain of multiplications). The
contraction occurs as the operations are actually performed. This type
of process, characterized by a chain of deferred operations, is called
a "recursive process". Carrying out this process requires that the
interpreter keep track of the operations to be performed later on. In
the computation of n!, the length of the chain of deferred
multiplications, and hence the amount of information needed to keep
track of it, grows linearly with n (is proportional to n), just like
the number of steps. Such a process is called a "linear recursive
process".
By contrast, the second process does not grow and shrink. At each
step, all we need to keep track of, for any n, are the current values
of the variables `product', `counter', and `maxcount'. We call this an "iterative
process". In general, an iterative process is one whose state can be
summarized by a fixed number of "state variables", together with a
fixed rule that describes how the state variables should be updated as
the process moves from state to state and an (optional) end test that
specifies conditions under which the process should terminate. In
computing n!, the number of steps required grows linearly with n. Such
a process is called a "linear iterative process".
The contrast between the two processes can be seen in another way.
In the iterative case, the program variables provide a complete
description of the state of the process at any point. If we stopped
the computation between steps, all we would need to do to resume the
computation is to supply the interpreter with the values of the three
program variables. Not so with the recursive process. In this case
there is some additional "hidden" information, maintained by the
interpreter and not contained in the program variables, which indicates
"where the process is" in negotiating the chain of deferred operations.
The longer the chain, the more information must be maintained.(2)
In contrasting iteration and recursion, we must be careful not to
confuse the notion of a recursive "process" with the notion of a
recursive "procedure". When we describe a procedure as recursive, we
are referring to the syntactic fact that the procedure definition
refers (either directly or indirectly) to the procedure itself. But
when we describe a process as following a pattern that is, say,
linearly recursive, we are speaking about how the process evolves, not
about the syntax of how a procedure is written. It may seem disturbing
that we refer to a recursive procedure such as `factiter' as
generating an iterative process. However, the process really is
iterative: Its state is captured completely by its three state
variables, and an interpreter need keep track of only three variables
in order to execute the process.
One reason that the distinction between process and procedure may be
confusing is that most implementations of common languages (including
Ada, Pascal, and C) are designed in such a way that the interpretation
of any recursive procedure consumes an amount of memory that grows with
the number of procedure calls, even when the process described is, in
principle, iterative. As a consequence, these languages can describe
iterative processes only by resorting to specialpurpose "looping
constructs" such as `do', `repeat', `until', `for', and `while'. The
implementation of Scheme we shall consider in *Note Chapter 5:: does
not share this defect. It will execute an iterative process in
constant space, even if the iterative process is described by a
recursive procedure. An implementation with this property is called "tailrecursive".
With a tailrecursive implementation, iteration can be expressed using
the ordinary procedure call mechanism, so that special iteration
constructs are useful only as syntactic sugar.(3)
*Exercise 1.9:* Each of the following two procedures defines a
method for adding two positive integers in terms of the procedures
`inc', which increments its argument by 1, and `dec', which
decrements its argument by 1.
(define (+ a b)
(if (= a 0)
b
(inc (+ (dec a) b))))
(define (+ a b)
(if (= a 0)
b
(+ (dec a) (inc b))))
Using the substitution model, illustrate the process generated by
each procedure in evaluating `(+ 4 5)'. Are these processes
iterative or recursive?
*Exercise 1.10:* The following procedure computes a mathematical
function called Ackermann's function.
(define (A x y)
(cond ((= y 0) 0)
((= x 0) (* 2 y))
((= y 1) 2)
(else (A ( x 1)
(A x ( y 1))))))
What are the values of the following expressions?
(A 1 10)
(A 2 4)
(A 3 3)
Consider the following procedures, where `A' is the procedure
defined above:
(define (f n) (A 0 n))
(define (g n) (A 1 n))
(define (h n) (A 2 n))
(define (k n) (* 5 n n))
Give concise mathematical definitions for the functions computed
by the procedures `f', `g', and `h' for positive integer values of
n. For example, `(k n)' computes 5n^2.
 Footnotes 
(1) In a real program we would probably use the block structure
introduced in the last section to hide the definition of `factiter':
(define (factorial n)
(define (iter product counter)
(if (> counter n)
product
(iter (* counter product)
(+ counter 1))))
(iter 1 1))
We avoided doing this here so as to minimize the number of things to
think about at once.
(2) When we discuss the implementation of procedures on register
machines in *Note Chapter 5::, we will see that any iterative process
can be realized "in hardware" as a machine that has a fixed set of
registers and no auxiliary memory. In contrast, realizing a recursive
process requires a machine that uses an auxiliary data structure known
as a "stack".
(3) Tail recursion has long been known as a compiler optimization
trick. A coherent semantic basis for tail recursion was provided by
Carl Hewitt (1977), who explained it in terms of the "messagepassing"
model of computation that we shall discuss in *Note Chapter 3::.
Inspired by this, Gerald Jay Sussman and Guy Lewis Steele Jr. (see
Steele 1975) constructed a tailrecursive interpreter for Scheme.
Steele later showed how tail recursion is a consequence of the natural
way to compile procedure calls (Steele 1977). The IEEE standard for
Scheme requires that Scheme implementations be tailrecursive.
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1.2.2 Tree Recursion

Another common pattern of computation is called "tree recursion". As
an example, consider computing the sequence of Fibonacci numbers, in
which each number is the sum of the preceding two:
0, 1, 1, 2, 3, 4, 8, 13, 21, ...
In general, the Fibonacci numbers can be defined by the rule
/
 0 if n = 0
Fib(n) = < 1 if n = 1
 Fib(n  1) + Fib(n  2) otherwise
\
We can immediately translate this definition into a recursive
procedure for computing Fibonacci numbers:
(define (fib n)
(cond ((= n 0) 0)
((= n 1) 1)
(else (+ (fib ( n 1))
(fib ( n 2))))))
*Figure 1.5:* The treerecursive process generated in computing
`(fib 5)'.
..<............ fib5 <..........
... ___________/ \___________ .
... / . ..... \ .
.. fib4 . . . . . fib3 .
.. ____/. \____ .. . __/ \__ .
.. / . . .. \ . .. / . . \ .
.. fib3 . . fib2 . . fib2 . . fib1 .
.. / . \ . . / \ . . / \ ... .  .
.. / . . \ . . / . \ . . / . \ . . 1 .
. fib2 . . fib1. .fib1 . fib0 . .fib1. . fib0 . . .
. / \ . .  . .  . .  . .  . .  . .>
V / . \ . 1 . . 1 . . 0 . . 1 . . 0 ..
. fib1 .. fib0.. . . . . . V . .. .
.  . .  . .> .>. . . ..>. .>
. 1 . . 0 .
. . . .
.>. ..
Consider the pattern of this computation. To compute `(fib 5)', we
compute `(fib 4)' and `(fib 3)'. To compute `(fib 4)', we compute
`(fib 3)' and `(fib 2)'. In general, the evolved process looks like a
tree, as shown in *Note Figure 15::. Notice that the branches split
into two at each level (except at the bottom); this reflects the fact
that the `fib' procedure calls itself twice each time it is invoked.
This procedure is instructive as a prototypical tree recursion, but
it is a terrible way to compute Fibonacci numbers because it does so
much redundant computation. Notice in *Note Figure 15:: that the
entire computation of `(fib 3)'almost half the workis duplicated.
In fact, it is not hard to show that the number of times the procedure
will compute `(fib 1)' or `(fib 0)' (the number of leaves in the above
tree, in general) is precisely _Fib_(n + 1). To get an idea of how bad
this is, one can show that the value of _Fib_(n) grows exponentially
with n. More precisely (see *Note Exercise 113::), _Fib_(n) is the
closest integer to [phi]^n /[sqrt](5), where
[phi] = (1 + [sqrt]5)/2 ~= 1.6180
is the "golden ratio", which satisfies the equation
[phi]^2 = [phi] + 1
Thus, the process uses a number of steps that grows exponentially
with the input. On the other hand, the space required grows only
linearly with the input, because we need keep track only of which nodes
are above us in the tree at any point in the computation. In general,
the number of steps required by a treerecursive process will be
proportional to the number of nodes in the tree, while the space
required will be proportional to the maximum depth of the tree.
We can also formulate an iterative process for computing the
Fibonacci numbers. The idea is to use a pair of integers a and b,
initialized to _Fib_(1) = 1 and _Fib_(0) = 0, and to repeatedly apply
the simultaneous transformations
a < a + b
b < a
It is not hard to show that, after applying this transformation n times,
a and b will be equal, respectively, to _Fib_(n + 1) and _Fib_(n).
Thus, we can compute Fibonacci numbers iteratively using the procedure
(define (fib n)
(fibiter 1 0 n))
(define (fibiter a b count)
(if (= count 0)
b
(fibiter (+ a b) a ( count 1))))
This second method for computing _Fib_(n) is a linear iteration. The
difference in number of steps required by the two methodsone linear in
n, one growing as fast as _Fib_(n) itselfis enormous, even for small
inputs.
One should not conclude from this that treerecursive processes are
useless. When we consider processes that operate on hierarchically
structured data rather than numbers, we will find that tree recursion
is a natural and powerful tool.(1) But even in numerical operations,
treerecursive processes can be useful in helping us to understand and
design programs. For instance, although the first `fib' procedure is
much less efficient than the second one, it is more straightforward,
being little more than a translation into Lisp of the definition of the
Fibonacci sequence. To formulate the iterative algorithm required
noticing that the computation could be recast as an iteration with
three state variables.
Example: Counting change
........................
It takes only a bit of cleverness to come up with the iterative
Fibonacci algorithm. In contrast, consider the following problem: How
many different ways can we make change of $ 1.00, given halfdollars,
quarters, dimes, nickels, and pennies? More generally, can we write a
procedure to compute the number of ways to change any given amount of
money?
This problem has a simple solution as a recursive procedure.
Suppose we think of the types of coins available as arranged in some
order. Then the following relation holds:
The number of ways to change amount a using n kinds of coins equals
* the number of ways to change amount a using all but the first kind
of coin, plus
* the number of ways to change amount a  d using all n kinds of
coins, where d is the denomination of the first kind of coin.
To see why this is true, observe that the ways to make change can be
divided into two groups: those that do not use any of the first kind of
coin, and those that do. Therefore, the total number of ways to make
change for some amount is equal to the number of ways to make change
for the amount without using any of the first kind of coin, plus the
number of ways to make change assuming that we do use the first kind of
coin. But the latter number is equal to the number of ways to make
change for the amount that remains after using a coin of the first kind.
Thus, we can recursively reduce the problem of changing a given
amount to the problem of changing smaller amounts using fewer kinds of
coins. Consider this reduction rule carefully, and convince yourself
that we can use it to describe an algorithm if we specify the following
degenerate cases:(2)
* If a is exactly 0, we should count that as 1 way to make change.
* If a is less than 0, we should count that as 0 ways to make change.
* If n is 0, we should count that as 0 ways to make change.
We can easily translate this description into a recursive procedure:
(define (countchange amount)
(cc amount 5))
(define (cc amount kindsofcoins)
(cond ((= amount 0) 1)
((or (< amount 0) (= kindsofcoins 0)) 0)
(else (+ (cc amount
( kindsofcoins 1))
(cc ( amount
(firstdenomination kindsofcoins))
kindsofcoins)))))
(define (firstdenomination kindsofcoins)
(cond ((= kindsofcoins 1) 1)
((= kindsofcoins 2) 5)
((= kindsofcoins 3) 10)
((= kindsofcoins 4) 25)
((= kindsofcoins 5) 50)))
(The `firstdenomination' procedure takes as input the number of
kinds of coins available and returns the denomination of the first
kind. Here we are thinking of the coins as arranged in order from
largest to smallest, but any order would do as well.) We can now
answer our original question about changing a dollar:
(countchange 100)
292
`Countchange' generates a treerecursive process with redundancies
similar to those in our first implementation of `fib'. (It will take
quite a while for that 292 to be computed.) On the other hand, it is
not obvious how to design a better algorithm for computing the result,
and we leave this problem as a challenge. The observation that a
treerecursive process may be highly inefficient but often easy to
specify and understand has led people to propose that one could get the
best of both worlds by designing a "smart compiler" that could
transform treerecursive procedures into more efficient procedures that
compute the same result.(3)
*Exercise 1.11:* A function f is defined by the rule that f(n) = n
if n<3 and f(n) = f(n  1) + 2f(n  2) + 3f(n  3) if n>= 3.
Write a procedure that computes f by means of a recursive process.
Write a procedure that computes f by means of an iterative
process.
*Exercise 1.12:* The following pattern of numbers is called "Pascal's
triangle".
1
1 1
1 2 1
1 3 3 1
1 4 6 4 1
The numbers at the edge of the triangle are all 1, and each number
inside the triangle is the sum of the two numbers above it.(4)
Write a procedure that computes elements of Pascal's triangle by
means of a recursive process.
*Exercise 1.13:* Prove that _Fib_(n) is the closest integer to
[phi]^n/[sqrt](5), where [phi] = (1 + [sqrt](5))/2. Hint: Let
[illegiblesymbol] = (1  [sqrt](5))/2. Use induction and the
definition of the Fibonacci numbers (see section *Note 122::) to
prove that _Fib_(n) = ([phi]^n  [illegiblesymbol]^n)/[sqrt](5).
 Footnotes 
(1) An example of this was hinted at in section *Note 113::: The
interpreter itself evaluates expressions using a treerecursive process.
(2) For example, work through in detail how the reduction rule
applies to the problem of making change for 10 cents using pennies and
nickels.
(3) One approach to coping with redundant computations is to arrange
matters so that we automatically construct a table of values as they
are computed. Each time we are asked to apply the procedure to some
argument, we first look to see if the value is already stored in the
table, in which case we avoid performing the redundant computation.
This strategy, known as "tabulation" or "memoization", can be
implemented in a straightforward way. Tabulation can sometimes be used
to transform processes that require an exponential number of steps
(such as `countchange') into processes whose space and time
requirements grow linearly with the input. See *Note Exercise 327::.
(4) The elements of Pascal's triangle are called the "binomial
coefficients", because the nth row consists of the coefficients of the
terms in the expansion of (x + y)^n. This pattern for computing the
coefficients appeared in Blaise Pascal's 1653 seminal work on
probability theory, `Traite' du triangle arithme'tique'. According to
Knuth (1973), the same pattern appears in the `Szuyuen Yu"chien'
("The Precious Mirror of the Four Elements"), published by the Chinese
mathematician Chu Shihchieh in 1303, in the works of the
twelfthcentury Persian poet and mathematician Omar Khayyam, and in the
works of the twelfthcentury Hindu mathematician Bha'scara A'cha'rya.
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1.2.3 Orders of Growth

The previous examples illustrate that processes can differ considerably
in the rates at which they consume computational resources. One
convenient way to describe this difference is to use the notion of "order
of growth" to obtain a gross measure of the resources required by a
process as the inputs become larger.
Let n be a parameter that measures the size of the problem, and let
R(n) be the amount of resources the process requires for a problem of
size n. In our previous examples we took n to be the number for which
a given function is to be computed, but there are other possibilities.
For instance, if our goal is to compute an approximation to the square
root of a number, we might take n to be the number of digits accuracy
required. For matrix multiplication we might take n to be the number
of rows in the matrices. In general there are a number of properties
of the problem with respect to which it will be desirable to analyze a
given process. Similarly, R(n) might measure the number of internal
storage registers used, the number of elementary machine operations
performed, and so on. In computers that do only a fixed number of
operations at a time, the time required will be proportional to the
number of elementary machine operations performed.
We say that R(n) has order of growth [theta](f(n)), written R(n) =
[theta](f(n)) (pronounced "theta of f(n)"), if there are positive
constants k_1 and k_2 independent of n such that
k_1 f(n) <= R(n) <= k_2 f(n)
for any sufficiently large value of n. (In other words, for large n,
the value R(n) is sandwiched between k_1f(n) and k_2f(n).)
For instance, with the linear recursive process for computing
factorial described in section *Note 121:: the number of steps grows
proportionally to the input n. Thus, the steps required for this
process grows as [theta](n). We also saw that the space required grows
as [theta](n). For the iterative factorial, the number of steps is
still [theta](n) but the space is [theta](1)that is, constant.(1) The
treerecursive Fibonacci computation requires [theta]([phi]^n) steps
and space [theta](n), where [phi] is the golden ratio described in
section *Note 122::.
Orders of growth provide only a crude description of the behavior of
a process. For example, a process requiring n^2 steps and a process
requiring 1000n^2 steps and a process requiring 3n^2 + 10n + 17 steps
all have [theta](n^2) order of growth. On the other hand, order of
growth provides a useful indication of how we may expect the behavior
of the process to change as we change the size of the problem. For a
[theta](n) (linear) process, doubling the size will roughly double the
amount of resources used. For an exponential process, each increment
in problem size will multiply the resource utilization by a constant
factor. In the remainder of section *Note 12:: we will examine two
algorithms whose order of growth is logarithmic, so that doubling the
problem size increases the resource requirement by a constant amount.
*Exercise 1.14:* Draw the tree illustrating the process generated
by the `countchange' procedure of section *Note 122:: in making
change for 11 cents. What are the orders of growth of the space
and number of steps used by this process as the amount to be
changed increases?
*Exercise 1.15:* The sine of an angle (specified in radians) can
be computed by making use of the approximation `sin' xapprox x if
x is sufficiently small, and the trigonometric identity
x x
sin x = 3 sin   4 sin^3 
3 3
to reduce the size of the argument of `sin'. (For purposes of this
exercise an angle is considered "sufficiently small" if its
magnitude is not greater than 0.1 radians.) These ideas are
incorporated in the following procedures:
(define (cube x) (* x x x))
(define (p x) ( (* 3 x) (* 4 (cube x))))
(define (sine angle)
(if (not (> (abs angle) 0.1))
angle
(p (sine (/ angle 3.0)))))
a. How many times is the procedure `p' applied when `(sine
12.15)' is evaluated?
b. What is the order of growth in space and number of steps (as
a function of a) used by the process generated by the `sine'
procedure when `(sine a)' is evaluated?
 Footnotes 
(1) These statements mask a great deal of oversimplification. For
instance, if we count process steps as "machine operations" we are
making the assumption that the number of machine operations needed to
perform, say, a multiplication is independent of the size of the
numbers to be multiplied, which is false if the numbers are
sufficiently large. Similar remarks hold for the estimates of space.
Like the design and description of a process, the analysis of a process
can be carried out at various levels of abstraction.
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1.2.4 Exponentiation

Consider the problem of computing the exponential of a given number.
We would like a procedure that takes as arguments a base b and a
positive integer exponent n and computes b^n. One way to do this is
via the recursive definition
b^n = b * b^(n  1)
b^0 = 1
which translates readily into the procedure
(define (expt b n)
(if (= n 0)
1
(* b (expt b ( n 1)))))
This is a linear recursive process, which requires [theta](n) steps
and [theta](n) space. Just as with factorial, we can readily formulate
an equivalent linear iteration:
(define (expt b n)
(exptiter b n 1))
(define (exptiter b counter product)
(if (= counter 0)
product
(exptiter b
( counter 1)
(* b product))))
This version requires [theta](n) steps and [theta](1) space.
We can compute exponentials in fewer steps by using successive
squaring. For instance, rather than computing b^8 as
b * (b * (b * (b * (b * (b * (b * b))))))
we can compute it using three multiplications:
b^2 = b * b
b^4 = b^2 * b^2
b^8 = b^4 * b^4
This method works fine for exponents that are powers of 2. We can
also take advantage of successive squaring in computing exponentials in
general if we use the rule
b^n = (b^(b/2))^2 if n is even
b^n = b * b^(n  1) if n is odd
We can express this method as a procedure:
(define (fastexpt b n)
(cond ((= n 0) 1)
((even? n) (square (fastexpt b (/ n 2))))
(else (* b (fastexpt b ( n 1))))))
where the predicate to test whether an integer is even is defined in
terms of the primitive procedure `remainder' by
(define (even? n)
(= (remainder n 2) 0))
The process evolved by `fastexpt' grows logarithmically with n in
both space and number of steps. To see this, observe that computing
b^(2n) using `fastexpt' requires only one more multiplication than
computing b^n. The size of the exponent we can compute therefore
doubles (approximately) with every new multiplication we are allowed.
Thus, the number of multiplications required for an exponent of n grows
about as fast as the logarithm of n to the base 2. The process has
[theta](`log' n) growth.(1)
The difference between [theta](`log' n) growth and [theta](n) growth
becomes striking as n becomes large. For example, `fastexpt' for n =
1000 requires only 14 multiplications.(2) It is also possible to use
the idea of successive squaring to devise an iterative algorithm that
computes exponentials with a logarithmic number of steps (see *Note
Exercise 116::), although, as is often the case with iterative
algorithms, this is not written down so straightforwardly as the
recursive algorithm.(3)
*Exercise 1.16:* Design a procedure that evolves an iterative
exponentiation process that uses successive squaring and uses a
logarithmic number of steps, as does `fastexpt'. (Hint: Using the
observation that (b^(n/2))^2 = (b^2)^(n/2), keep, along with the
exponent n and the base b, an additional state variable a, and
define the state transformation in such a way that the product a
b^n is unchanged from state to state. At the beginning of the
process a is taken to be 1, and the answer is given by the value
of a at the end of the process. In general, the technique of
defining an "invariant quantity" that remains unchanged from state
to state is a powerful way to think about the design of iterative
algorithms.)
*Exercise 1.17:* The exponentiation algorithms in this section are
based on performing exponentiation by means of repeated
multiplication. In a similar way, one can perform integer
multiplication by means of repeated addition. The following
multiplication procedure (in which it is assumed that our language
can only add, not multiply) is analogous to the `expt' procedure:
(define (* a b)
(if (= b 0)
0
(+ a (* a ( b 1)))))
This algorithm takes a number of steps that is linear in `b'. Now
suppose we include, together with addition, operations `double',
which doubles an integer, and `halve', which divides an (even)
integer by 2. Using these, design a multiplication procedure
analogous to `fastexpt' that uses a logarithmic number of steps.
*Exercise 1.18:* Using the results of *Note Exercise 116:: and
*Note Exercise 117::, devise a procedure that generates an
iterative process for multiplying two integers in terms of adding,
doubling, and halving and uses a logarithmic number of steps.(4)
*Exercise 1.19:* There is a clever algorithm for computing the
Fibonacci numbers in a logarithmic number of steps. Recall the
transformation of the state variables a and b in the `fibiter'
process of section *Note 122::: a < a + b and b < a. Call
this transformation T, and observe that applying T over and over
again n times, starting with 1 and 0, produces the pair _Fib_(n +
1) and _Fib_(n). In other words, the Fibonacci numbers are
produced by applying T^n, the nth power of the transformation T,
starting with the pair (1,0). Now consider T to be the special
case of p = 0 and q = 1 in a family of transformations T_(pq),
where T_(pq) transforms the pair (a,b) according to a < bq + aq +
ap and b < bp + aq. Show that if we apply such a transformation
T_(pq) twice, the effect is the same as using a single
transformation T_(p'q') of the same form, and compute p' and q' in
terms of p and q. This gives us an explicit way to square these
transformations, and thus we can compute T^n using successive
squaring, as in the `fastexpt' procedure. Put this all together
to complete the following procedure, which runs in a logarithmic
number of steps:(5)
(define (fib n)
(fibiter 1 0 0 1 n))
(define (fibiter a b p q count)
(cond ((= count 0) b)
((even? count)
(fibiter a
b
; compute p'
; compute q'
(/ count 2)))
(else (fibiter (+ (* b q) (* a q) (* a p))
(+ (* b p) (* a q))
p
q
( count 1)))))
 Footnotes 
(1) More precisely, the number of multiplications required is equal
to 1 less than the log base 2 of n plus the number of ones in the
binary representation of n. This total is always less than twice the
log base 2 of n. The arbitrary constants k_1 and k_2 in the definition
of order notation imply that, for a logarithmic process, the base to
which logarithms are taken does not matter, so all such processes are
described as [theta](`log' n).
(2) You may wonder why anyone would care about raising numbers to
the 1000th power. See section *Note 126::.
(3) This iterative algorithm is ancient. It appears in the
`Chandahsutra' by A'cha'rya Pingala, written before 200 B.C. See Knuth
1981, section 4.6.3, for a full discussion and analysis of this and
other methods of exponentiation.
(4) This algorithm, which is sometimes known as the "Russian peasant
method" of multiplication, is ancient. Examples of its use are found
in the Rhind Papyrus, one of the two oldest mathematical documents in
existence, written about 1700 B.C. (and copied from an even older
document) by an Egyptian scribe named A'hmose.
(5) This exercise was suggested to us by Joe Stoy, based on an
example in Kaldewaij 1990.
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1.2.5 Greatest Common Divisors

The greatest common divisor (GCD) of two integers a and b is defined to
be the largest integer that divides both a and b with no remainder.
For example, the GCD of 16 and 28 is 4. In *Note Chapter 2::, when we
investigate how to implement rationalnumber arithmetic, we will need
to be able to compute GCDs in order to reduce rational numbers to
lowest terms. (To reduce a rational number to lowest terms, we must
divide both the numerator and the denominator by their GCD. For
example, 16/28 reduces to 4/7.) One way to find the GCD of two
integers is to factor them and search for common factors, but there is
a famous algorithm that is much more efficient.
The idea of the algorithm is based on the observation that, if r is
the remainder when a is divided by b, then the common divisors of a and
b are precisely the same as the common divisors of b and r. Thus, we
can use the equation
GCD(a,b) = GCD(b,r)
to successively reduce the problem of computing a GCD to the problem of
computing the GCD of smaller and smaller pairs of integers. For
example,
GCD(206,40) = GCD(40,6)
= GCD(6,4)
= GCD(4,2)
= GCD(2,0)
= 2
reduces GCD(206,40) to GCD(2,0), which is 2. It is possible to show
that starting with any two positive integers and performing repeated
reductions will always eventually produce a pair where the second
number is 0. Then the GCD is the other number in the pair. This
method for computing the GCD is known as Algorithm "Euclid's
Algorithm".(1)
It is easy to express Euclid's Algorithm as a procedure:
(define (gcd a b)
(if (= b 0)
a
(gcd b (remainder a b))))
This generates an iterative process, whose number of steps grows as
the logarithm of the numbers involved.
The fact that the number of steps required by Euclid's Algorithm has
logarithmic growth bears an interesting relation to the Fibonacci
numbers:
*Lame''s Theorem:* If Euclid's Algorithm requires k steps to
compute the GCD of some pair, then the smaller number in the pair
must be greater than or equal to the kth Fibonacci number.(2)
We can use this theorem to get an orderofgrowth estimate for
Euclid's Algorithm. Let n be the smaller of the two inputs to the
procedure. If the process takes k steps, then we must have n>= _Fib_(k)
approx [phi]^k/[sqrt](5). Therefore the number of steps k grows as the
logarithm (to the base [phi]) of n. Hence, the order of growth is
[theta](`log' n).
*Exercise 1.20:* The process that a procedure generates is of
course dependent on the rules used by the interpreter. As an
example, consider the iterative `gcd' procedure given above.
Suppose we were to interpret this procedure using normalorder
evaluation, as discussed in section *Note 115::. (The
normalorderevaluation rule for `if' is described in *Note
Exercise 15::.) Using the substitution method (for normal
order), illustrate the process generated in evaluating `(gcd 206
40)' and indicate the `remainder' operations that are actually
performed. How many `remainder' operations are actually performed
in the normalorder evaluation of `(gcd 206 40)'? In the
applicativeorder evaluation?
 Footnotes 
(1) Euclid's Algorithm is so called because it appears in Euclid's
`Elements' (Book 7, ca. 300 B.C.). According to Knuth (1973), it can
be considered the oldest known nontrivial algorithm. The ancient
Egyptian method of multiplication (*Note Exercise 118::) is surely
older, but, as Knuth explains, Euclid's algorithm is the oldest known
to have been presented as a general algorithm, rather than as a set of
illustrative examples.
(2) This theorem was proved in 1845 by Gabriel Lame', a French
mathematician and engineer known chiefly for his contributions to
mathematical physics. To prove the theorem, we consider pairs (a_k
,b_k), where a_k>= b_k, for which Euclid's Algorithm terminates in k
steps. The proof is based on the claim that, if (a_(k+1), b_(k+1)) >
(a_k, b_k) > (a_(k1), b_(k1)) are three successive pairs in the
reduction process, then we must have b_(k+1)>= b_k + b_(k1). To
verify the claim, consider that a reduction step is defined by applying
the transformation a_(k1) = b_k, b_(k1) = remainder of a_k divided by
b_k. The second equation means that a_k = qb_k + b_(k1) for some
positive integer q. And since q must be at least 1 we have a_k = qb_k
+ b_(k1) >= b_k + b_(k1). But in the previous reduction step we have
b_(k+1) = a_k. Therefore, b_(k+1) = a_k>= b_k + b_(k1). This verifies
the claim. Now we can prove the theorem by induction on k, the number
of steps that the algorithm requires to terminate. The result is true
for k = 1, since this merely requires that b be at least as large as
_Fib_(1) = 1. Now, assume that the result is true for all integers
less than or equal to k and establish the result for k + 1. Let
(a_(k+1), b_(k+1)) > (a_k, b_k) > (a_k1), b_(k1)) be successive
pairs in the reduction process. By our induction hypotheses, we have
b_(k1) >= _Fib_(k  1) and b_k >= _Fib_(k). Thus, applying the claim
we just proved together with the definition of the Fibonacci numbers
gives b_(k+1) >= b_k + b_(k1) >= _Fib_(k) + _Fib_(k  1) = _Fib_(k +
1), which completes the proof of Lame''s Theorem.
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1.2.6 Example: Testing for Primality

This section describes two methods for checking the primality of an
integer n, one with order of growth [theta](_[sqrt]_(n)), and a
"probabilistic" algorithm with order of growth [theta](`log' n). The
exercises at the end of this section suggest programming projects based
on these algorithms.
Searching for divisors
......................
Since ancient times, mathematicians have been fascinated by problems
concerning prime numbers, and many people have worked on the problem of
determining ways to test if numbers are prime. One way to test if a
number is prime is to find the number's divisors. The following
program finds the smallest integral divisor (greater than 1) of a given
number n. It does this in a straightforward way, by testing n for
divisibility by successive integers starting with 2.
(define (smallestdivisor n)
(finddivisor n 2))
(define (finddivisor n testdivisor)
(cond ((> (square testdivisor) n) n)
((divides? testdivisor n) testdivisor)
(else (finddivisor n (+ testdivisor 1)))))
(define (divides? a b)
(= (remainder b a) 0))
We can test whether a number is prime as follows: n is prime if and
only if n is its own smallest divisor.
(define (prime? n)
(= n (smallestdivisor n)))
The end test for `finddivisor' is based on the fact that if n is not
prime it must have a divisor less than or equal to _[sqrt]_(n).(1)
This means that the algorithm need only test divisors between 1 and
_[sqrt]_(n). Consequently, the number of steps required to identify n
as prime will have order of growth [theta](_[sqrt]_(n)).
The Fermat test
...............
The [theta](`log' n) primality test is based on a result from number
theory known as Fermat's Little Theorem.(2)
*Fermat's Little Theorem:* If n is a prime number and a is any
positive integer less than n, then a raised to the nth power is
congruent to a modulo n.
(Two numbers are said to be "congruent modulo" n if they both have
the same remainder when divided by n. The remainder of a number a when
divided by n is also referred to as the "remainder of" a "modulo" n, or
simply as a "modulo" n.)
If n is not prime, then, in general, most of the numbers a< n will
not satisfy the above relation. This leads to the following algorithm
for testing primality: Given a number n, pick a random number a < n and
compute the remainder of a^n modulo n. If the result is not equal to
a, then n is certainly not prime. If it is a, then chances are good
that n is prime. Now pick another random number a and test it with the
same method. If it also satisfies the equation, then we can be even
more confident that n is prime. By trying more and more values of a,
we can increase our confidence in the result. This algorithm is known
as the Fermat test.
To implement the Fermat test, we need a procedure that computes the
exponential of a number modulo another number:
(define (expmod base exp m)
(cond ((= exp 0) 1)
((even? exp)
(remainder (square (expmod base (/ exp 2) m))
m))
(else
(remainder (* base (expmod base ( exp 1) m))
m))))
This is very similar to the `fastexpt' procedure of section *Note
124::. It uses successive squaring, so that the number of steps
grows logarithmically with the exponent.(3)
The Fermat test is performed by choosing at random a number a
between 1 and n  1 inclusive and checking whether the remainder modulo
n of the nth power of a is equal to a. The random number a is chosen
using the procedure `random', which we assume is included as a primitive
in Scheme. `Random' returns a nonnegative integer less than its integer
input. Hence, to obtain a random number between 1 and n  1, we call
`random' with an input of n  1 and add 1 to the result:
(define (fermattest n)
(define (tryit a)
(= (expmod a n n) a))
(tryit (+ 1 (random ( n 1)))))
The following procedure runs the test a given number of times, as
specified by a parameter. Its value is true if the test succeeds every
time, and false otherwise.
(define (fastprime? n times)
(cond ((= times 0) true)
((fermattest n) (fastprime? n ( times 1)))
(else false)))
Probabilistic methods
.....................
The Fermat test differs in character from most familiar algorithms, in
which one computes an answer that is guaranteed to be correct. Here,
the answer obtained is only probably correct. More precisely, if n
ever fails the Fermat test, we can be certain that n is not prime. But
the fact that n passes the test, while an extremely strong indication,
is still not a guarantee that n is prime. What we would like to say is
that for any number n, if we perform the test enough times and find
that n always passes the test, then the probability of error in our
primality test can be made as small as we like.
Unfortunately, this assertion is not quite correct. There do exist
numbers that fool the Fermat test: numbers n that are not prime and yet
have the property that a^n is congruent to a modulo n for all integers
a < n. Such numbers are extremely rare, so the Fermat test is quite
reliable in practice.(4)
There are variations of the Fermat test that cannot be fooled. In
these tests, as with the Fermat method, one tests the primality of an
integer n by choosing a random integer a a b)
0
(+ a (sumintegers (+ a 1) b))))
The second computes the sum of the cubes of the integers in the
given range:
(define (sumcubes a b)
(if (> a b)
0
(+ (cube a) (sumcubes (+ a 1) b))))
The third computes the sum of a sequence of terms in the series
1 1 1
 +  +  + ...
1 * 3 5 * 7 9 * 11
which converges to [pi]/8 (very slowly):(1)
(define (pisum a b)
(if (> a b)
0
(+ (/ 1.0 (* a (+ a 2))) (pisum (+ a 4) b))))
These three procedures clearly share a common underlying pattern.
They are for the most part identical, differing only in the name of the
procedure, the function of `a' used to compute the term to be added,
and the function that provides the next value of `a'. We could
generate each of the procedures by filling in slots in the same
template:
(define ( a b)
(if (> a b)
0
(+ ( a)
( ( a) b))))
The presence of such a common pattern is strong evidence that there
is a useful abstraction waiting to be brought to the surface. Indeed,
mathematicians long ago identified the abstraction of "summation of a
series" and invented "sigma notation," for example
b

> f(n) = f(a) + ... + f(b)

n=a
to express this concept. The power of sigma notation is that it allows
mathematicians to deal with the concept of summation itself rather than
only with particular sumsfor example, to formulate general results
about sums that are independent of the particular series being summed.
Similarly, as program designers, we would like our language to be
powerful enough so that we can write a procedure that expresses the
concept of summation itself rather than only procedures that compute
particular sums. We can do so readily in our procedural language by
taking the common template shown above and transforming the "slots"
into formal parameters:
(define (sum term a next b)
(if (> a b)
0
(+ (term a)
(sum term (next a) next b))))
Notice that `sum' takes as its arguments the lower and upper bounds
`a' and `b' together with the procedures `term' and `next'. We can use
`sum' just as we would any procedure. For example, we can use it
(along with a procedure `inc' that increments its argument by 1) to
define `sumcubes':
(define (inc n) (+ n 1))
(define (sumcubes a b)
(sum cube a inc b))
Using this, we can compute the sum of the cubes of the integers from
1 to 10:
(sumcubes 1 10)
3025
With the aid of an identity procedure to compute the term, we can
define `sumintegers' in terms of `sum':
(define (identity x) x)
(define (sumintegers a b)
(sum identity a inc b))
Then we can add up the integers from 1 to 10:
(sumintegers 1 10)
55
We can also define `pisum' in the same way:(2)
(define (pisum a b)
(define (piterm x)
(/ 1.0 (* x (+ x 2))))
(define (pinext x)
(+ x 4))
(sum piterm a pinext b))
Using these procedures, we can compute an approximation to [pi]:
(* 8 (pisum 1 1000))
3.139592655589783
Once we have `sum', we can use it as a building block in formulating
further concepts. For instance, the definite integral of a function f
between the limits a and b can be approximated numerically using the
formula
/b / / dx \ / dx \ / dx \ \
 f =  f a +   + f a + dx +   + f a + 2dx +   + ... dx
/a \ \ 2 / \ 2 / \ 2 / /
for small values of dx. We can express this directly as a procedure:
(define (integral f a b dx)
(define (adddx x) (+ x dx))
(* (sum f (+ a (/ dx 2.0)) adddx b)
dx))
(integral cube 0 1 0.01)
.24998750000000042
(integral cube 0 1 0.001)
.249999875000001
(The exact value of the integral of `cube' between 0 and 1 is 1/4.)
*Exercise 1.29:* Simpson's Rule is a more accurate method of
numerical integration than the method illustrated above. Using
Simpson's Rule, the integral of a function f between a and b is
approximated as
h
 (y_0 + 4y_1 + 2y_2 + 4y_3 + 2y_4 + ... + 2y_(n2) + 4y_(n1) + y_n)
3
where h = (b  a)/n, for some even integer n, and y_k = f(a + kh).
(Increasing n increases the accuracy of the approximation.)
Define a procedure that takes as arguments f, a, b, and n and
returns the value of the integral, computed using Simpson's Rule.
Use your procedure to integrate `cube' between 0 and 1 (with n =
100 and n = 1000), and compare the results to those of the
`integral' procedure shown above.
*Exercise 1.30:* The `sum' procedure above generates a linear
recursion. The procedure can be rewritten so that the sum is
performed iteratively. Show how to do this by filling in the
missing expressions in the following definition:
(define (sum term a next b)
(define (iter a result)
(if
(iter )))
(iter ))
*Exercise 1.31:*
a. The `sum' procedure is only the simplest of a vast number of
similar abstractions that can be captured as higherorder
procedures.(3) Write an analogous procedure called `product'
that returns the product of the values of a function at
points over a given range. Show how to define `factorial' in
terms of `product'. Also use `product' to compute
approximations to [pi] using the formula(4)
pi 2 * 4 * 4 * 6 * 6 * 8 ...
 = 
4 3 * 3 * 5 * 5 * 7 * 7 ...
b. If your `product' procedure generates a recursive process,
write one that generates an iterative process. If it
generates an iterative process, write one that generates a
recursive process.
*Exercise 1.32:*
a. Show that `sum' and `product' (*Note Exercise 131::) are
both special cases of a still more general notion called
`accumulate' that combines a collection of terms, using some
general accumulation function:
(accumulate combiner nullvalue term a next b)
`Accumulate' takes as arguments the same term and range
specifications as `sum' and `product', together with a
`combiner' procedure (of two arguments) that specifies how
the current term is to be combined with the accumulation of
the preceding terms and a `nullvalue' that specifies what
base value to use when the terms run out. Write `accumulate'
and show how `sum' and `product' can both be defined as
simple calls to `accumulate'.
b. If your `accumulate' procedure generates a recursive process,
write one that generates an iterative process. If it
generates an iterative process, write one that generates a
recursive process.
*Exercise 1.33:* You can obtain an even more general version of
`accumulate' (*Note Exercise 132::) by introducing the notion of
a "filter" on the terms to be combined. That is, combine only
those terms derived from values in the range that satisfy a
specified condition. The resulting `filteredaccumulate'
abstraction takes the same arguments as accumulate, together with
an additional predicate of one argument that specifies the filter.
Write `filteredaccumulate' as a procedure. Show how to express
the following using `filteredaccumulate':
a. the sum of the squares of the prime numbers in the interval a
to b (assuming that you have a `prime?' predicate already
written)
b. the product of all the positive integers less than n that are
relatively prime to n (i.e., all positive integers i < n such
that GCD(i,n) = 1).
 Footnotes 
(1) This series, usually written in the equivalent form ([pi]/4) = 1
 (1/3) + (1/5)  (1/7) + ..., is due to Leibniz. We'll see how to use
this as the basis for some fancy numerical tricks in section *Note
353::.
(2) Notice that we have used block structure (section *Note 118::)
to embed the definitions of `pinext' and `piterm' within `pisum',
since these procedures are unlikely to be useful for any other purpose.
We will see how to get rid of them altogether in section *Note 132::.
(3) The intent of *Note Exercise 131:: through *Note Exercise
133:: is to demonstrate the expressive power that is attained by using
an appropriate abstraction to consolidate many seemingly disparate
operations. However, though accumulation and filtering are elegant
ideas, our hands are somewhat tied in using them at this point since we
do not yet have data structures to provide suitable means of
combination for these abstractions. We will return to these ideas in
section *Note 223:: when we show how to use "sequences" as interfaces
for combining filters and accumulators to build even more powerful
abstractions. We will see there how these methods really come into
their own as a powerful and elegant approach to designing programs.
(4) This formula was discovered by the seventeenthcentury English
mathematician John Wallis.
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1.3.2 Constructing Procedures Using `Lambda'

In using `sum' as in section *Note 131::, it seems terribly awkward to
have to define trivial procedures such as `piterm' and `pinext' just
so we can use them as arguments to our higherorder procedure. Rather
than define `pinext' and `piterm', it would be more convenient to
have a way to directly specify "the procedure that returns its input
incremented by 4" and "the procedure that returns the reciprocal of its
input times its input plus 2." We can do this by introducing the
special form `lambda', which creates procedures. Using `lambda' we can
describe what we want as
(lambda (x) (+ x 4))
and
(lambda (x) (/ 1.0 (* x (+ x 2))))
Then our `pisum' procedure can be expressed without defining any
auxiliary procedures as
(define (pisum a b)
(sum (lambda (x) (/ 1.0 (* x (+ x 2))))
a
(lambda (x) (+ x 4))
b))
Again using `lambda', we can write the `integral' procedure without
having to define the auxiliary procedure `adddx':
(define (integral f a b dx)
(* (sum f
(+ a (/ dx 2.0))
(lambda (x) (+ x dx))
b)
dx))
In general, `lambda' is used to create procedures in the same way as
`define', except that no name is specified for the procedure:
(lambda () )
The resulting procedure is just as much a procedure as one that is
created using `define'. The only difference is that it has not been
associated with any name in the environment. In fact,
(define (plus4 x) (+ x 4))
is equivalent to
(define plus4 (lambda (x) (+ x 4)))
We can read a `lambda' expression as follows:
(lambda (x) (+ x 4))
    
the procedure of an argument x that adds x and 4
Like any expression that has a procedure as its value, a `lambda'
expression can be used as the operator in a combination such as
((lambda (x y z) (+ x y (square z))) 1 2 3)
12
or, more generally, in any context where we would normally use a
procedure name.(1)
Using `let' to create local variables
.....................................
Another use of `lambda' is in creating local variables. We often need
local variables in our procedures other than those that have been bound
as formal parameters. For example, suppose we wish to compute the
function
f(x,y) = x(1 + xy)^2 + y(1  y) + (1 + xy)(1  y)
which we could also express as
a = 1 + xy
b = 1  y
f(x,y) = xa^2 + yb + ab
In writing a procedure to compute f, we would like to include as
local variables not only x and y but also the names of intermediate
quantities like a and b. One way to accomplish this is to use an
auxiliary procedure to bind the local variables:
(define (f x y)
(define (fhelper a b)
(+ (* x (square a))
(* y b)
(* a b)))
(fhelper (+ 1 (* x y))
( 1 y)))
Of course, we could use a `lambda' expression to specify an anonymous
procedure for binding our local variables. The body of `f' then
becomes a single call to that procedure:
(define (f x y)
((lambda (a b)
(+ (* x (square a))
(* y b)
(* a b)))
(+ 1 (* x y))
( 1 y)))
This construct is so useful that there is a special form called
`let' to make its use more convenient. Using `let', the `f' procedure
could be written as
(define (f x y)
(let ((a (+ 1 (* x y)))
(b ( 1 y)))
(+ (* x (square a))
(* y b)
(* a b))))
The general form of a `let' expression is
(let (( )
( )
...
( ))
)
which can be thought of as saying
let have the value and
have the value and
...
have the value
in
The first part of the `let' expression is a list of nameexpression
pairs. When the `let' is evaluated, each name is associated with the
value of the corresponding expression. The body of the `let' is
evaluated with these names bound as local variables. The way this
happens is that the `let' expression is interpreted as an alternate
syntax for
((lambda ( ... )
)
...
)
No new mechanism is required in the interpreter in order to provide
local variables. A `let' expression is simply syntactic sugar for the
underlying `lambda' application.
We can see from this equivalence that the scope of a variable
specified by a `let' expression is the body of the `let'. This implies
that:
* `Let' allows one to bind variables as locally as possible to where
they are to be used. For example, if the value of `x' is 5, the
value of the expression
(+ (let ((x 3))
(+ x (* x 10)))
x)
is 38. Here, the `x' in the body of the `let' is 3, so the value
of the `let' expression is 33. On the other hand, the `x' that is
the second argument to the outermost `+' is still 5.
* The variables' values are computed outside the `let'. This
matters when the expressions that provide the values for the local
variables depend upon variables having the same names as the local
variables themselves. For example, if the value of `x' is 2, the
expression
(let ((x 3)
(y (+ x 2)))
(* x y))
will have the value 12 because, inside the body of the `let', `x'
will be 3 and `y' will be 4 (which is the outer `x' plus 2).
Sometimes we can use internal definitions to get the same effect as
with `let'. For example, we could have defined the procedure `f' above
as
(define (f x y)
(define a (+ 1 (* x y)))
(define b ( 1 y))
(+ (* x (square a))
(* y b)
(* a b)))
We prefer, however, to use `let' in situations like this and to use
internal `define' only for internal procedures.(2)
*Exercise 1.34:* Suppose we define the procedure
(define (f g)
(g 2))
Then we have
(f square)
4
(f (lambda (z) (* z (+ z 1))))
6
What happens if we (perversely) ask the interpreter to evaluate
the combination `(f f)'? Explain.
 Footnotes 
(1) It would be clearer and less intimidating to people learning
Lisp if a name more obvious than `lambda', such as `makeprocedure',
were used. But the convention is firmly entrenched. The notation is
adopted from the [lambda] calculus, a mathematical formalism introduced
by the mathematical logician Alonzo Church (1941). Church developed
the [lambda] calculus to provide a rigorous foundation for studying the
notions of function and function application. The [lambda] calculus
has become a basic tool for mathematical investigations of the
semantics of programming languages.
(2) Understanding internal definitions well enough to be sure a
program means what we intend it to mean requires a more elaborate model
of the evaluation process than we have presented in this chapter. The
subtleties do not arise with internal definitions of procedures,
however. We will return to this issue in section *Note 416::, after
we learn more about evaluation.
File: sicp.info, Node: 133, Next: 134, Prev: 132, Up: 13
1.3.3 Procedures as General Methods

We introduced compound procedures in section *Note 114:: as a
mechanism for abstracting patterns of numerical operations so as to
make them independent of the particular numbers involved. With
higherorder procedures, such as the `integral' procedure of section
*Note 131::, we began to see a more powerful kind of abstraction:
procedures used to express general methods of computation, independent
of the particular functions involved. In this section we discuss two
more elaborate examplesgeneral methods for finding zeros and fixed
points of functionsand show how these methods can be expressed
directly as procedures.
Finding roots of equations by the halfinterval method
......................................................
The "halfinterval method" is a simple but powerful technique for
finding roots of an equation f(x) = 0, where f is a continuous
function. The idea is that, if we are given points a and b such that
f(a) < 0 < f(b), then f must have at least one zero between a and b.
To locate a zero, let x be the average of a and b and compute f(x). If
f(x) > 0, then f must have a zero between a and x. If f(x) < 0, then f
must have a zero between x and b. Continuing in this way, we can
identify smaller and smaller intervals on which f must have a zero.
When we reach a point where the interval is small enough, the process
stops. Since the interval of uncertainty is reduced by half at each
step of the process, the number of steps required grows as
[theta](`log'( L/T)), where L is the length of the original interval
and T is the error tolerance (that is, the size of the interval we will
consider "small enough"). Here is a procedure that implements this
strategy:
(define (search f negpoint pospoint)
(let ((midpoint (average negpoint pospoint)))
(if (closeenough? negpoint pospoint)
midpoint
(let ((testvalue (f midpoint)))
(cond ((positive? testvalue)
(search f negpoint midpoint))
((negative? testvalue)
(search f midpoint pospoint))
(else midpoint))))))
We assume that we are initially given the function f together with
points at which its values are negative and positive. We first compute
the midpoint of the two given points. Next we check to see if the
given interval is small enough, and if so we simply return the midpoint
as our answer. Otherwise, we compute as a test value the value of f at
the midpoint. If the test value is positive, then we continue the
process with a new interval running from the original negative point to
the midpoint. If the test value is negative, we continue with the
interval from the midpoint to the positive point. Finally, there is
the possibility that the test value is 0, in which case the midpoint is
itself the root we are searching for.
To test whether the endpoints are "close enough" we can use a
procedure similar to the one used in section *Note 117:: for
computing square roots:(1)
(define (closeenough? x y)
(< (abs ( x y)) 0.001))
`Search' is awkward to use directly, because we can accidentally
give it points at which f's values do not have the required sign, in
which case we get a wrong answer. Instead we will use `search' via the
following procedure, which checks to see which of the endpoints has a
negative function value and which has a positive value, and calls the
`search' procedure accordingly. If the function has the same sign on
the two given points, the halfinterval method cannot be used, in which
case the procedure signals an error.(2)
(define (halfintervalmethod f a b)
(let ((avalue (f a))
(bvalue (f b)))
(cond ((and (negative? avalue) (positive? bvalue))
(search f a b))
((and (negative? bvalue) (positive? avalue))
(search f b a))
(else
(error "Values are not of opposite sign" a b)))))
The following example uses the halfinterval method to approximate
[pi] as the root between 2 and 4 of `sin' x = 0:
(halfintervalmethod sin 2.0 4.0)
3.14111328125
Here is another example, using the halfinterval method to search
for a root of the equation x^3  2x  3 = 0 between 1 and 2:
(halfintervalmethod (lambda (x) ( (* x x x) (* 2 x) 3))
1.0
2.0)
1.89306640625
Finding fixed points of functions
.................................
A number x is called a "fixed point" of a function f if x satisfies the
equation f(x) = x. For some functions f we can locate a fixed point by
beginning with an initial guess and applying f repeatedly,
f(x), f(f(x), (f(f(f(x))))
until the value does not change very much. Using this idea, we can
devise a procedure `fixedpoint' that takes as inputs a function and an
initial guess and produces an approximation to a fixed point of the
function. We apply the function repeatedly until we find two
successive values whose difference is less than some prescribed
tolerance:
(define tolerance 0.00001)
(define (fixedpoint f firstguess)
(define (closeenough? v1 v2)
(< (abs ( v1 v2)) tolerance))
(define (try guess)
(let ((next (f guess)))
(if (closeenough? guess next)
next
(try next))))
(try firstguess))
For example, we can use this method to approximate the fixed point
of the cosine function, starting with 1 as an initial approximation:(3)
(fixedpoint cos 1.0)
.7390822985224023
Similarly, we can find a solution to the equation y = `sin' y +
`cos' y:
(fixedpoint (lambda (y) (+ (sin y) (cos y)))
1.0)
1.2587315962971173
The fixedpoint process is reminiscent of the process we used for
finding square roots in section *Note 117::. Both are based on the
idea of repeatedly improving a guess until the result satisfies some
criterion. In fact, we can readily formulate the squareroot
computation as a fixedpoint search. Computing the square root of some
number x requires finding a y such that y^2 = x. Putting this equation
into the equivalent form y = x/y, we recognize that we are looking for
a fixed point of the function(4) y > x/y, and we can therefore try to
compute square roots as
(define (sqrt x)
(fixedpoint (lambda (y) (/ x y))
1.0))
Unfortunately, this fixedpoint search does not converge. Consider
an initial guess y_1. The next guess is y_2 = x/y_1 and the next guess
is y_3 = x/y_2 = x/(x/y_1) = y_1. This results in an infinite loop in
which the two guesses y_1 and y_2 repeat over and over, oscillating
about the answer.
One way to control such oscillations is to prevent the guesses from
changing so much. Since the answer is always between our guess y and
x/y, we can make a new guess that is not as far from y as x/y by
averaging y with x/y, so that the next guess after y is (1/2)(y + x/y)
instead of x/y. The process of making such a sequence of guesses is
simply the process of looking for a fixed point of y > (1/2)(y + x/y):
(define (sqrt x)
(fixedpoint (lambda (y) (average y (/ x y)))
1.0))
(Note that y = (1/2)(y + x/y) is a simple transformation of the
equation y = x/y; to derive it, add y to both sides of the equation and
divide by 2.)
With this modification, the squareroot procedure works. In fact,
if we unravel the definitions, we can see that the sequence of
approximations to the square root generated here is precisely the same
as the one generated by our original squareroot procedure of section
*Note 117::. This approach of averaging successive approximations to
a solution, a technique we that we call "average damping", often aids
the convergence of fixedpoint searches.
*Exercise 1.35:* Show that the golden ratio [phi] (section *Note
122::) is a fixed point of the transformation x > 1 + 1/x, and
use this fact to compute [phi] by means of the `fixedpoint'
procedure.
*Exercise 1.36:* Modify `fixedpoint' so that it prints the
sequence of approximations it generates, using the `newline' and
`display' primitives shown in *Note Exercise 122::. Then find a
solution to x^x = 1000 by finding a fixed point of x >
`log'(1000)/`log'(x). (Use Scheme's primitive `log' procedure,
which computes natural logarithms.) Compare the number of steps
this takes with and without average damping. (Note that you
cannot start `fixedpoint' with a guess of 1, as this would cause
division by `log'(1) = 0.)
*Exercise 1.37:*
a. An infinite "continued fraction" is an expression of the form
N_1
f = 
N_2
D_1 + 
N_3
D_2 + 
D_3 + ...
As an example, one can show that the infinite continued
fraction expansion with the n_i and the D_i all equal to 1
produces 1/[phi], where [phi] is the golden ratio (described
in section *Note 122::). One way to approximate an
infinite continued fraction is to truncate the expansion
after a given number of terms. Such a truncationa
socalled finite continued fraction "kterm finite continued
fraction"has the form
N_1

N_2
D_1 + 
... N_K
+ 
D_K
Suppose that `n' and `d' are procedures of one argument (the
term index i) that return the n_i and D_i of the terms of the
continued fraction. Define a procedure `contfrac' such that
evaluating `(contfrac n d k)' computes the value of the
kterm finite continued fraction. Check your procedure by
approximating 1/[phi] using
(contfrac (lambda (i) 1.0)
(lambda (i) 1.0)
k)
for successive values of `k'. How large must you make `k' in
order to get an approximation that is accurate to 4 decimal
places?
b. If your `contfrac' procedure generates a recursive process,
write one that generates an iterative process. If it
generates an iterative process, write one that generates a
recursive process.
*Exercise 1.38:* In 1737, the Swiss mathematician Leonhard Euler
published a memoir `De Fractionibus Continuis', which included a
continued fraction expansion for e  2, where e is the base of the
natural logarithms. In this fraction, the n_i are all 1, and the
D_i are successively 1, 2, 1, 1, 4, 1, 1, 6, 1, 1, 8, .... Write
a program that uses your `contfrac' procedure from *Note Exercise
137:: to approximate e, based on Euler's expansion.
*Exercise 1.39:* A continued fraction representation of the
tangent function was published in 1770 by the German mathematician
J.H. Lambert:
x
tan x = 
x^2
1  
x^2
3  
5  ...
where x is in radians. Define a procedure `(tancf x k)' that
computes an approximation to the tangent function based on
Lambert's formula. `K' specifies the number of terms to compute,
as in *Note Exercise 137::.
 Footnotes 
(1) We have used 0.001 as a representative "small" number to
indicate a tolerance for the acceptable error in a calculation. The
appropriate tolerance for a real calculation depends upon the problem
to be solved and the limitations of the computer and the algorithm.
This is often a very subtle consideration, requiring help from a
numerical analyst or some other kind of magician.
(2) This can be accomplished using `error', which takes as arguments
a number of items that are printed as error messages.
(3) Try this during a boring lecture: Set your calculator to radians
mode and then repeatedly press the `cos' button until you obtain the
fixed point.
(4) > (pronounced "maps to") is the mathematician's way of writing
`lambda'. y > x/y means `(lambda(y) (/ x y))', that is, the function
whose value at y is x/y.
File: sicp.info, Node: 134, Prev: 133, Up: 13
1.3.4 Procedures as Returned Values

The above examples demonstrate how the ability to pass procedures as
arguments significantly enhances the expressive power of our
programming language. We can achieve even more expressive power by
creating procedures whose returned values are themselves procedures.
We can illustrate this idea by looking again at the fixedpoint
example described at the end of section *Note 133::. We formulated a
new version of the squareroot procedure as a fixedpoint search,
starting with the observation that [sqrt]x is a fixedpoint of the
function y > x/y. Then we used average damping to make the
approximations converge. Average damping is a useful general technique
in itself. Namely, given a function f, we consider the function whose
value at x is equal to the average of x and f(x).
We can express the idea of average damping by means of the following
procedure:
(define (averagedamp f)
(lambda (x) (average x (f x))))
`Averagedamp' is a procedure that takes as its argument a procedure
`f' and returns as its value a procedure (produced by the `lambda')
that, when applied to a number `x', produces the average of `x' and `(f
x)'. For example, applying `averagedamp' to the `square' procedure
produces a procedure whose value at some number x is the average of x
and x^2. Applying this resulting procedure to 10 returns the average
of 10 and 100, or 55:(1)
((averagedamp square) 10)
55
Using `averagedamp', we can reformulate the squareroot procedure as
follows:
(define (sqrt x)
(fixedpoint (averagedamp (lambda (y) (/ x y)))
1.0))
Notice how this formulation makes explicit the three ideas in the
method: fixedpoint search, average damping, and the function y > x/y.
It is instructive to compare this formulation of the squareroot method
with the original version given in section *Note 117::. Bear in mind
that these procedures express the same process, and notice how much
clearer the idea becomes when we express the process in terms of these
abstractions. In general, there are many ways to formulate a process
as a procedure. Experienced programmers know how to choose procedural
formulations that are particularly perspicuous, and where useful
elements of the process are exposed as separate entities that can be
reused in other applications. As a simple example of reuse, notice
that the cube root of x is a fixed point of the function y > x/y^2,
so we can immediately generalize our squareroot procedure to one that
extracts cube roots:(2)
(define (cuberoot x)
(fixedpoint (averagedamp (lambda (y) (/ x (square y))))
1.0))
Newton's method
...............
When we first introduced the squareroot procedure, in section *Note
117::, we mentioned that this was a special case of "Newton's
method". If x > g(x) is a differentiable function, then a solution
of the equation g(x) = 0 is a fixed point of the function x > f(x)
where
g(x)
f(x) = x  
Dg(x)
and Dg(x) is the derivative of g evaluated at x. Newton's method is
the use of the fixedpoint method we saw above to approximate a
solution of the equation by finding a fixed point of the function f.(3)
For many functions g and for sufficiently good initial guesses for x,
Newton's method converges very rapidly to a solution of g(x) = 0.(4)
In order to implement Newton's method as a procedure, we must first
express the idea of derivative. Note that "derivative," like average
damping, is something that transforms a function into another function.
For instance, the derivative of the function x > x^3 is the function
x > 3x^2. In general, if g is a function and dx is a small number,
then the derivative Dg of g is the function whose value at any number x
is given (in the limit of small dx) by
g(x + dx)  g(x)
Dg(c) = 
dx
Thus, we can express the idea of derivative (taking dx to be, say,
0.00001) as the procedure
(define (deriv g)
(lambda (x)
(/ ( (g (+ x dx)) (g x))
dx)))
along with the definition
(define dx 0.00001)
Like `averagedamp', `deriv' is a procedure that takes a procedure as
argument and returns a procedure as value. For example, to approximate
the derivative of x > x^3 at 5 (whose exact value is 75) we can
evaluate
(define (cube x) (* x x x))
((deriv cube) 5)
75.00014999664018
With the aid of `deriv', we can express Newton's method as a
fixedpoint process:
(define (newtontransform g)
(lambda (x)
( x (/ (g x) ((deriv g) x)))))
(define (newtonsmethod g guess)
(fixedpoint (newtontransform g) guess))
The `newtontransform' procedure expresses the formula at the
beginning of this section, and `newtonsmethod' is readily defined in
terms of this. It takes as arguments a procedure that computes the
function for which we want to find a zero, together with an initial
guess. For instance, to find the square root of x, we can use Newton's
method to find a zero of the function y > y^2  x starting with an
initial guess of 1.(5)
This provides yet another form of the squareroot procedure:
(define (sqrt x)
(newtonsmethod (lambda (y) ( (square y) x))
1.0))
Abstractions and firstclass procedures
.......................................
We've seen two ways to express the squareroot computation as an
instance of a more general method, once as a fixedpoint search and
once using Newton's method. Since Newton's method was itself expressed
as a fixedpoint process, we actually saw two ways to compute square
roots as fixed points. Each method begins with a function and finds a
fixed point of some transformation of the function. We can express
this general idea itself as a procedure:
(define (fixedpointoftransform g transform guess)
(fixedpoint (transform g) guess))
This very general procedure takes as its arguments a procedure `g'
that computes some function, a procedure that transforms `g', and an
initial guess. The returned result is a fixed point of the transformed
function.
Using this abstraction, we can recast the first squareroot
computation from this section (where we look for a fixed point of the
averagedamped version of y > x/y) as an instance of this general
method:
(define (sqrt x)
(fixedpointoftransform (lambda (y) (/ x y))
averagedamp
1.0))
Similarly, we can express the second squareroot computation from
this section (an instance of Newton's method that finds a fixed point
of the Newton transform of y > y^2  x) as
(define (sqrt x)
(fixedpointoftransform (lambda (y) ( (square y) x))
newtontransform
1.0))
We began section *Note 13:: with the observation that compound
procedures are a crucial abstraction mechanism, because they permit us
to express general methods of computing as explicit elements in our
programming language. Now we've seen how higherorder procedures
permit us to manipulate these general methods to create further
abstractions.
As programmers, we should be alert to opportunities to identify the
underlying abstractions in our programs and to build upon them and
generalize them to create more powerful abstractions. This is not to
say that one should always write programs in the most abstract way
possible; expert programmers know how to choose the level of
abstraction appropriate to their task. But it is important to be able
to think in terms of these abstractions, so that we can be ready to
apply them in new contexts. The significance of higherorder
procedures is that they enable us to represent these abstractions
explicitly as elements in our programming language, so that they can be
handled just like other computational elements.
In general, programming languages impose restrictions on the ways in
which computational elements can be manipulated. Elements with the
fewest restrictions are said to have "firstclass" status. Some of the
"rights and privileges" of firstclass elements are:(6)
* They may be named by variables.
* They may be passed as arguments to procedures.
* They may be returned as the results of procedures.
* They may be included in data structures.(7)
Lisp, unlike other common programming languages, awards procedures
full firstclass status. This poses challenges for efficient
implementation, but the resulting gain in expressive power is
enormous.(8)
*Exercise 1.40:* Define a procedure `cubic' that can be used
together with the `newtonsmethod' procedure in expressions of the
form
(newtonsmethod (cubic a b c) 1)
to approximate zeros of the cubic x^3 + ax^2 + bx + c.
*Exercise 1.41:* Define a procedure `double' that takes a
procedure of one argument as argument and returns a procedure that
applies the original procedure twice. For example, if `inc' is a
procedure that adds 1 to its argument, then `(double inc)' should
be a procedure that adds 2. What value is returned by
(((double (double double)) inc) 5)
*Exercise 1.42:* Let f and g be two oneargument functions. The "composition"
f after g is defined to be the function x > f(g(x)). Define a
procedure `compose' that implements composition. For example, if
`inc' is a procedure that adds 1 to its argument,
((compose square inc) 6)
49
*Exercise 1.43:* If f is a numerical function and n is a positive
integer, then we can form the nth repeated application of f, which
is defined to be the function whose value at x is
f(f(...(f(x))...)). For example, if f is the function x > x +
1, then the nth repeated application of f is the function x > x
+ n. If f is the operation of squaring a number, then the nth
repeated application of f is the function that raises its argument
to the 2^nth power. Write a procedure that takes as inputs a
procedure that computes f and a positive integer n and returns the
procedure that computes the nth repeated application of f. Your
procedure should be able to be used as follows:
((repeated square 2) 5)
625
Hint: You may find it convenient to use `compose' from *Note
Exercise 142::.
*Exercise 1.44:* The idea of "smoothing" a function is an
important concept in signal processing. If f is a function and dx
is some small number, then the smoothed version of f is the
function whose value at a point x is the average of f(x  dx),
f(x), and f(x + dx). Write a procedure `smooth' that takes as
input a procedure that computes f and returns a procedure that
computes the smoothed f. It is sometimes valuable to repeatedly
smooth a function (that is, smooth the smoothed function, and so
on) to obtained the "nfold smoothed function". Show how to
generate the nfold smoothed function of any given function using
`smooth' and `repeated' from *Note Exercise 143::.
*Exercise 1.45:* We saw in section *Note 133:: that attempting
to compute square roots by naively finding a fixed point of y >
x/y does not converge, and that this can be fixed by average
damping. The same method works for finding cube roots as fixed
points of the averagedamped y > x/y^2. Unfortunately, the
process does not work for fourth rootsa single average damp is
not enough to make a fixedpoint search for y > x/y^3 converge.
On the other hand, if we average damp twice (i.e., use the average
damp of the average damp of y > x/y^3) the fixedpoint search
does converge. Do some experiments to determine how many average
damps are required to compute nth roots as a fixedpoint search
based upon repeated average damping of y > x/y^(n1). Use this
to implement a simple procedure for computing nth roots using
`fixedpoint', `averagedamp', and the `repeated' procedure of
*Note Exercise 143::. Assume that any arithmetic operations you
need are available as primitives.
*Exercise 1.46:* Several of the numerical methods described in
this chapter are instances of an extremely general computational
strategy known as "iterative improvement". Iterative improvement
says that, to compute something, we start with an initial guess
for the answer, test if the guess is good enough, and otherwise
improve the guess and continue the process using the improved
guess as the new guess. Write a procedure `iterativeimprove'
that takes two procedures as arguments: a method for telling
whether a guess is good enough and a method for improving a guess.
`Iterativeimprove' should return as its value a procedure that
takes a guess as argument and keeps improving the guess until it
is good enough. Rewrite the `sqrt' procedure of section *Note
117:: and the `fixedpoint' procedure of section *Note 133::
in terms of `iterativeimprove'.
 Footnotes 
(1) Observe that this is a combination whose operator is itself a
combination. *Note Exercise 14:: already demonstrated the ability to
form such combinations, but that was only a toy example. Here we begin
to see the real need for such combinationswhen applying a procedure
that is obtained as the value returned by a higherorder procedure.
(2) See *Note Exercise 145:: for a further generalization.
(3) Elementary calculus books usually describe Newton's method in
terms of the sequence of approximations x_(n+1) = x_n  g(x_n)/Dg(x_n).
Having language for talking about processes and using the idea of
fixed points simplifies the description of the method.
(4) Newton's method does not always converge to an answer, but it can
be shown that in favorable cases each iteration doubles the
numberofdigits accuracy of the approximation to the solution. In
such cases, Newton's method will converge much more rapidly than the
halfinterval method.
(5) For finding square roots, Newton's method converges rapidly to
the correct solution from any starting point.
(6) The notion of firstclass status of programminglanguage
elements is due to the British computer scientist Christopher Strachey
(19161975).
(7) We'll see examples of this after we introduce data structures in
*Note Chapter 2::.
(8) The major implementation cost of firstclass procedures is that
allowing procedures to be returned as values requires reserving storage
for a procedure's free variables even while the procedure is not
executing. In the Scheme implementation we will study in section *Note
41::, these variables are stored in the procedure's environment.
File: sicp.info, Node: Chapter 2, Next: Chapter 3, Prev: Chapter 1, Up: Top
2 Building Abstractions with Data
*********************************
We now come to the decisive step of mathematical abstraction: we
forget about what the symbols stand for. ...[The mathematician]
need not be idle; there are many operations which he may carry out
with these symbols, without ever having to look at the things they
stand for.
Hermann Weyl, `The Mathematical Way of Thinking'
We concentrated in *Note Chapter 1:: on computational processes and
on the role of procedures in program design. We saw how to use
primitive data (numbers) and primitive operations (arithmetic
operations), how to combine procedures to form compound procedures
through composition, conditionals, and the use of parameters, and how
to abstract procedures by using `define'. We saw that a procedure can
be regarded as a pattern for the local evolution of a process, and we
classified, reasoned about, and performed simple algorithmic analyses of
some common patterns for processes as embodied in procedures. We also
saw that higherorder procedures enhance the power of our language by
enabling us to manipulate, and thereby to reason in terms of, general
methods of computation. This is much of the essence of programming.
In this chapter we are going to look at more complex data. All the
procedures in *Note Chapter 1:: operate on simple numerical data, and
simple data are not sufficient for many of the problems we wish to
address using computation. Programs are typically designed to model
complex phenomena, and more often than not one must construct
computational objects that have several parts in order to model
realworld phenomena that have several aspects. Thus, whereas our
focus in *Note Chapter 1:: was on building abstractions by combining
procedures to form compound procedures, we turn in this chapter to
another key aspect of any programming language: the means it provides
for building abstractions by combining data objects to form "compound
data".
Why do we want compound data in a programming language? For the
same reasons that we want compound procedures: to elevate the
conceptual level at which we can design our programs, to increase the
modularity of our designs, and to enhance the expressive power of our
language. Just as the ability to define procedures enables us to deal
with processes at a higher conceptual level than that of the primitive
operations of the language, the ability to construct compound data
objects enables us to deal with data at a higher conceptual level than
that of the primitive data objects of the language.
Consider the task of designing a system to perform arithmetic with
rational numbers. We could imagine an operation `addrat' that takes
two rational numbers and produces their sum. In terms of simple data,
a rational number can be thought of as two integers: a numerator and a
denominator. Thus, we could design a program in which each rational
number would be represented by two integers (a numerator and a
denominator) and where `addrat' would be implemented by two procedures
(one producing the numerator of the sum and one producing the
denominator). But this would be awkward, because we would then need to
explicitly keep track of which numerators corresponded to which
denominators. In a system intended to perform many operations on many
rational numbers, such bookkeeping details would clutter the programs
substantially, to say nothing of what they would do to our minds. It
would be much better if we could "glue together" a numerator and
denominator to form a paira "compound data object"that our programs
could manipulate in a way that would be consistent with regarding a
rational number as a single conceptual unit.
The use of compound data also enables us to increase the modularity
of our programs. If we can manipulate rational numbers directly as
objects in their own right, then we can separate the part of our
program that deals with rational numbers per se from the details of how
rational numbers may be represented as pairs of integers. The general
technique of isolating the parts of a program that deal with how data
objects are represented from the parts of a program that deal with how
data objects are used is a powerful design methodology called "data
abstraction". We will see how data abstraction makes programs much
easier to design, maintain, and modify.
The use of compound data leads to a real increase in the expressive
power of our programming language. Consider the idea of forming a
"linear combination" ax + by. We might like to write a procedure that
would accept a, b, x, and y as arguments and return the value of ax +
by. This presents no difficulty if the arguments are to be numbers,
because we can readily define the procedure
(define (linearcombination a b x y)
(+ (* a x) (* b y)))
But suppose we are not concerned only with numbers. Suppose we
would like to express, in procedural terms, the idea that one can form
linear combinations whenever addition and multiplication are
definedfor rational numbers, complex numbers, polynomials, or
whatever. We could express this as a procedure of the form
(define (linearcombination a b x y)
(add (mul a x) (mul b y)))
where `add' and `mul' are not the primitive procedures `+' and `*' but
rather more complex things that will perform the appropriate operations
for whatever kinds of data we pass in as the arguments `a', `b', `x',
and `y'. The key point is that the only thing `linearcombination'
should need to know about `a', `b', `x', and `y' is that the procedures
`add' and `mul' will perform the appropriate manipulations. From the
perspective of the procedure `linearcombination', it is irrelevant
what `a', `b', `x', and `y' are and even more irrelevant how they might
happen to be represented in terms of more primitive data. This same
example shows why it is important that our programming language provide
the ability to manipulate compound objects directly: Without this,
there is no way for a procedure such as `linearcombination' to pass
its arguments along to `add' and `mul' without having to know their
detailed structure.(1)
We begin this chapter by implementing the rationalnumber arithmetic
system mentioned above. This will form the background for our
discussion of compound data and data abstraction. As with compound
procedures, the main issue to be addressed is that of abstraction as a
technique for coping with complexity, and we will see how data
abstraction enables us to erect suitable "abstraction barriers" between
different parts of a program.
We will see that the key to forming compound data is that a
programming language should provide some kind of "glue" so that data
objects can be combined to form more complex data objects. There are
many possible kinds of glue. Indeed, we will discover how to form
compound data using no special "data" operations at all, only
procedures. This will further blur the distinction between "procedure"
and "data," which was already becoming tenuous toward the end of *Note
Chapter 1::. We will also explore some conventional techniques for
representing sequences and trees. One key idea in dealing with
compound data is the notion of "closure"that the glue we use for
combining data objects should allow us to combine not only primitive
data objects, but compound data objects as well. Another key idea is
that compound data objects can serve as "conventional interfaces" for
combining program modules in mixandmatch ways. We illustrate some of
these ideas by presenting a simple graphics language that exploits
closure.
We will then augment the representational power of our language by
introducing "symbolic expressions"data whose elementary parts can be
arbitrary symbols rather than only numbers. We explore various
alternatives for representing sets of objects. We will find that, just
as a given numerical function can be computed by many different
computational processes, there are many ways in which a given data
structure can be represented in terms of simpler objects, and the
choice of representation can have significant impact on the time and
space requirements of processes that manipulate the data. We will
investigate these ideas in the context of symbolic differentiation, the
representation of sets, and the encoding of information.
Next we will take up the problem of working with data that may be
represented differently by different parts of a program. This leads to
the need to implement "generic operations", which must handle many
different types of data. Maintaining modularity in the presence of
generic operations requires more powerful abstraction barriers than can
be erected with simple data abstraction alone. In particular, we
introduce programming "datadirected programming" as a technique that
allows individual data representations to be designed in isolation and
then combined "additively" (i.e., without modification). To illustrate
the power of this approach to system design, we close the chapter by
applying what we have learned to the implementation of a package for
performing symbolic arithmetic on polynomials, in which the
coefficients of the polynomials can be integers, rational numbers,
complex numbers, and even other polynomials.
* Menu:
* 21:: Introduction to Data Abstraction
* 22:: Hierarchical Data and the Closure Property
* 23:: Symbolic Data
* 24:: Multiple Representations for Abstract Data
* 25:: Systems with Generic Operations
 Footnotes 
(1) The ability to directly manipulate procedures provides an
analogous increase in the expressive power of a programming language.
For example, in section *Note 131:: we introduced the `sum'
procedure, which takes a procedure `term' as an argument and computes
the sum of the values of `term' over some specified interval. In order
to define `sum', it is crucial that we be able to speak of a procedure
such as `term' as an entity in its own right, without regard for how
`term' might be expressed with more primitive operations. Indeed, if
we did not have the notion of "a procedure," it is doubtful that we
would ever even think of the possibility of defining an operation such
as `sum'. Moreover, insofar as performing the summation is concerned,
the details of how `term' may be constructed from more primitive
operations are irrelevant.
File: sicp.info, Node: 21, Next: 22, Prev: Chapter 2, Up: Chapter 2
2.1 Introduction to Data Abstraction
====================================
In section *Note 118::, we noted that a procedure used as an element
in creating a more complex procedure could be regarded not only as a
collection of particular operations but also as a procedural
abstraction. That is, the details of how the procedure was implemented
could be suppressed, and the particular procedure itself could be
replaced by any other procedure with the same overall behavior. In
other words, we could make an abstraction that would separate the way
the procedure would be used from the details of how the procedure would
be implemented in terms of more primitive procedures. The analogous
notion for compound data is called "data abstraction". Data
abstraction is a methodology that enables us to isolate how a compound
data object is used from the details of how it is constructed from more
primitive data objects.
The basic idea of data abstraction is to structure the programs that
are to use compound data objects so that they operate on "abstract
data." That is, our programs should use data in such a way as to make
no assumptions about the data that are not strictly necessary for
performing the task at hand. At the same time, a "concrete" data
representation is defined independent of the programs that use the
data. The interface between these two parts of our system will be a
set of procedures, called "selectors" and "constructors", that
implement the abstract data in terms of the concrete representation. To
illustrate this technique, we will consider how to design a set of
procedures for manipulating rational numbers.
* Menu:
* 211:: Example: Arithmetic Operations for Rational Numbers
* 212:: Abstraction Barriers
* 213:: What Is Meant by Data?
* 214:: Extended Exercise: Interval Arithmetic
File: sicp.info, Node: 211, Next: 212, Prev: 21, Up: 21
2.1.1 Example: Arithmetic Operations for Rational Numbers

Suppose we want to do arithmetic with rational numbers. We want to be
able to add, subtract, multiply, and divide them and to test whether
two rational numbers are equal.
Let us begin by assuming that we already have a way of constructing
a rational number from a numerator and a denominator. We also assume
that, given a rational number, we have a way of extracting (or
selecting) its numerator and its denominator. Let us further assume
that the constructor and selectors are available as procedures:
* `(makerat )' returns therational number whose numerator is
the integer `' and whose denominator is the integer `'.
* `(numer )' returns the numerator of the rational number `'.
* `(denom )' returns the denominator of the rational number `'.
We are using here a powerful strategy of synthesis: "wishful
thinking". We haven't yet said how a rational number is represented,
or how the procedures `numer', `denom', and `makerat' should be
implemented. Even so, if we did have these three procedures, we could
then add, subtract, multiply, divide, and test equality by using the
following relations:
n_1 n_2 n_1 d_2 + n_2 d_1
 +  = 
d_1 d_2 d_1 d_2
n_1 n_2 n_1 d_2  n_2 d_1
   = 
d_1 d_2 d_1 d_2
n_1 n_2 n_1 n_2
 *  = 
d_1 d_2 d_1 d_2
n_1 / d_1 n_1 d_2
 = 
n_2 / d_2 d_1 n_2
n_1 n_2
 =  if and only if n_1 d_2 = n_2 d_1
d_1 d_2
We can express these rules as procedures:
(define (addrat x y)
(makerat (+ (* (numer x) (denom y))
(* (numer y) (denom x)))
(* (denom x) (denom y))))
(define (subrat x y)
(makerat ( (* (numer x) (denom y))
(* (numer y) (denom x)))
(* (denom x) (denom y))))
(define (mulrat x y)
(makerat (* (numer x) (numer y))
(* (denom x) (denom y))))
(define (divrat x y)
(makerat (* (numer x) (denom y))
(* (denom x) (numer y))))
(define (equalrat? x y)
(= (* (numer x) (denom y))
(* (numer y) (denom x))))
Now we have the operations on rational numbers defined in terms of
the selector and constructor procedures `numer', `denom', and
`makerat'. But we haven't yet defined these. What we need is some
way to glue together a numerator and a denominator to form a rational
number.
Pairs
.....
To enable us to implement the concrete level of our data abstraction,
our language provides a compound structure called a "pair", which can be
constructed with the primitive procedure `cons'. This procedure takes
two arguments and returns a compound data object that contains the two
arguments as parts. Given a pair, we can extract the parts using the
primitive procedures `car' and `cdr'.(1) Thus, we can use `cons',
`car', and `cdr' as follows:
(define x (cons 1 2))
(car x)
1
(cdr x)
2
Notice that a pair is a data object that can be given a name and
manipulated, just like a primitive data object. Moreover, `cons' can
be used to form pairs whose elements are pairs, and so on:
(define x (cons 1 2))
(define y (cons 3 4))
(define z (cons x y))
(car (car z))
1
(car (cdr z))
3
In section *Note 22:: we will see how this ability to combine pairs
means that pairs can be used as generalpurpose building blocks to
create all sorts of complex data structures. The single compounddata
primitive "pair", implemented by the procedures `cons', `car', and
`cdr', is the only glue we need. Data objects constructed from pairs
are called "liststructured" data.
Representing rational numbers
.............................
Pairs offer a natural way to complete the rationalnumber system.
Simply represent a rational number as a pair of two integers: a
numerator and a denominator. Then `makerat', `numer', and `denom' are
readily implemented as follows:(2)
(define (makerat n d) (cons n d))
(define (numer x) (car x))
(define (denom x) (cdr x))
Also, in order to display the results of our computations, we can
print rational numbers by printing the numerator, a slash, and the
denominator:(3)
(define (printrat x)
(newline)
(display (numer x))
(display "/")
(display (denom x)))
Now we can try our rationalnumber procedures:
(define onehalf (makerat 1 2))
(printrat onehalf)
1/2
(define onethird (makerat 1 3))
(printrat (addrat onehalf onethird))
5/6
(printrat (mulrat onehalf onethird))
1/6
(printrat (addrat onethird onethird))
6/9
As the final example shows, our rationalnumber implementation does
not reduce rational numbers to lowest terms. We can remedy this by
changing `makerat'. If we have a `gcd' procedure like the one in
section *Note 125:: that produces the greatest common divisor of two
integers, we can use `gcd' to reduce the numerator and the denominator
to lowest terms before constructing the pair:
(define (makerat n d)
(let ((g (gcd n d)))
(cons (/ n g) (/ d g))))
Now we have
(printrat (addrat onethird onethird))
2/3
as desired. This modification was accomplished by changing the
constructor `makerat' without changing any of the procedures (such as
`addrat' and `mulrat') that implement the actual operations.
*Exercise 2.1:* Define a better version of `makerat' that handles
both positive and negative arguments. `Makerat' should normalize
the sign so that if the rational number is positive, both the
numerator and denominator are positive, and if the rational number
is negative, only the numerator is negative.
 Footnotes 
(1) The name `cons' stands for "construct." The names `car' and
`cdr' derive from the original implementation of Lisp on the IBM 704.
That machine had an addressing scheme that allowed one to reference the
"address" and "decrement" parts of a memory location. `Car' stands for
"Contents of Address part of Register" and `cdr' (pronounced
"coulder") stands for "Contents of Decrement part of Register."
(2) Another way to define the selectors and constructor is
(define makerat cons)
(define numer car)
(define denom cdr)
The first definition associates the name `makerat' with the value
of the expression `cons', which is the primitive procedure that
constructs pairs. Thus `makerat' and `cons' are names for the same
primitive constructor.
Defining selectors and constructors in this way is efficient:
Instead of `makerat' _calling_ `cons', `makerat' _is_ `cons', so
there is only one procedure called, not two, when `makerat' is called.
On the other hand, doing this defeats debugging aids that trace
procedure calls or put breakpoints on procedure calls: You may want to
watch `makerat' being called, but you certainly don't want to watch
every call to `cons'.
We have chosen not to use this style of definition in this book.
(3) `Display' is the Scheme primitive for printing data. The Scheme
primitive `newline' starts a new line for printing. Neither of these
procedures returns a useful value, so in the uses of `printrat' below,
we show only what `printrat' prints, not what the interpreter prints
as the value returned by `printrat'.
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2.1.2 Abstraction Barriers

Before continuing with more examples of compound data and data
abstraction, let us consider some of the issues raised by the
rationalnumber example. We defined the rationalnumber operations in
terms of a constructor `makerat' and selectors `numer' and `denom'.
In general, the underlying idea of data abstraction is to identify for
each type of data object a basic set of operations in terms of which
all manipulations of data objects of that type will be expressed, and
then to use only those operations in manipulating the data.
We can envision the structure of the rationalnumber system as shown
in figure *Note Figure 21::. The horizontal lines represent barriers
"abstraction barriers" that isolate different "levels" of the system.
At each level, the barrier separates the programs (above) that use the
data abstraction from the programs (below) that implement the data
abstraction. Programs that use rational numbers manipulate them solely
in terms of the procedures supplied "for public use" by the
rationalnumber package: `addrat', `subrat', `mulrat', `divrat',
and `equalrat?'. These, in turn, are implemented solely in terms of
the constructor and selectors `makerat', `numer', and `denom', which
themselves are implemented in terms of pairs. The details of how pairs
are implemented are irrelevant to the rest of the rationalnumber
package so long as pairs can be manipulated by the use of `cons',
`car', and `cdr'. In effect, procedures at each level are the
interfaces that define the abstraction barriers and connect the
different levels.
*Figure 2.1:* Dataabstraction barriers in the rationalnumber
package.
++
 Programs that use rational numbers 
++
Rational numbers in promblem domain
++
 addrat subrat ... 
++
Rational numbers as numerators and denominators
++
 makerat numer denom 
++
Rational numbers as pairs
++
 cons car cdr 
++
However pairs are implemented
This simple idea has many advantages. One advantage is that it
makes programs much easier to maintain and to modify. Any complex data
structure can be represented in a variety of ways with the primitive
data structures provided by a programming language. Of course, the
choice of representation influences the programs that operate on it;
thus, if the representation were to be changed at some later time, all
such programs might have to be modified accordingly. This task could
be timeconsuming and expensive in the case of large programs unless
the dependence on the representation were to be confined by design to a
very few program modules.
For example, an alternate way to address the problem of reducing
rational numbers to lowest terms is to perform the reduction whenever
we access the parts of a rational number, rather than when we construct
it. This leads to different constructor and selector procedures:
(define (makerat n d)
(cons n d))
(define (numer x)
(let ((g (gcd (car x) (cdr x))))
(/ (car x) g)))
(define (denom x)
(let ((g (gcd (car x) (cdr x))))
(/ (cdr x) g)))
The difference between this implementation and the previous one lies
in when we compute the `gcd'. If in our typical use of rational
numbers we access the numerators and denominators of the same rational
numbers many times, it would be preferable to compute the `gcd' when
the rational numbers are constructed. If not, we may be better off
waiting until access time to compute the `gcd'. In any case, when we
change from one representation to the other, the procedures `addrat',
`subrat', and so on do not have to be modified at all.
Constraining the dependence on the representation to a few interface
procedures helps us design programs as well as modify them, because it
allows us to maintain the flexibility to consider alternate
implementations. To continue with our simple example, suppose we are
designing a rationalnumber package and we can't decide initially
whether to perform the `gcd' at construction time or at selection time.
The dataabstraction methodology gives us a way to defer that decision
without losing the ability to make progress on the rest of the system.
*Exercise 2.2:* Consider the problem of representing line segments
in a plane. Each segment is represented as a pair of points: a
starting point and an ending point. Define a constructor
`makesegment' and selectors `startsegment' and `endsegment'
that define the representation of segments in terms of points.
Furthermore, a point can be represented as a pair of numbers: the
x coordinate and the y coordinate. Accordingly, specify a
constructor `makepoint' and selectors `xpoint' and `ypoint'
that define this representation. Finally, using your selectors
and constructors, define a procedure `midpointsegment' that takes
a line segment as argument and returns its midpoint (the point
whose coordinates are the average of the coordinates of the
endpoints). To try your procedures, you'll need a way to print
points:
(define (printpoint p)
(newline)
(display "(")
(display (xpoint p))
(display ",")
(display (ypoint p))
(display ")"))
*Exercise 2.3:* Implement a representation for rectangles in a
plane. (Hint: You may want to make use of *Note Exercise 22::.)
In terms of your constructors and selectors, create procedures
that compute the perimeter and the area of a given rectangle. Now
implement a different representation for rectangles. Can you
design your system with suitable abstraction barriers, so that the
same perimeter and area procedures will work using either
representation?
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2.1.3 What Is Meant by Data?

We began the rationalnumber implementation in section *Note 211:: by
implementing the rationalnumber operations `addrat', `subrat', and
so on in terms of three unspecified procedures: `makerat', `numer',
and `denom'. At that point, we could think of the operations as being
defined in terms of data objectsnumerators, denominators, and rational
numberswhose behavior was specified by the latter three procedures.
But exactly what is meant by "data"? It is not enough to say
"whatever is implemented by the given selectors and constructors."
Clearly, not every arbitrary set of three procedures can serve as an
appropriate basis for the rationalnumber implementation. We need to
guarantee that, if we construct a rational number `x' from a pair of
integers `n' and `d', then extracting the `numer' and the `denom' of
`x' and dividing them should yield the same result as dividing `n' by
`d'. In other words, `makerat', `numer', and `denom' must satisfy the
condition that, for any integer `n' and any nonzero integer `d', if
`x' is (`makerat n d'), then
(numer x) n
 = 
(denom x) d
In fact, this is the only condition `makerat', `numer', and `denom'
must fulfill in order to form a suitable basis for a rationalnumber
representation. In general, we can think of data as defined by some
collection of selectors and constructors, together with specified
conditions that these procedures must fulfill in order to be a valid
representation.(1)
This point of view can serve to define not only "highlevel" data
objects, such as rational numbers, but lowerlevel objects as well.
Consider the notion of a pair, which we used in order to define our
rational numbers. We never actually said what a pair was, only that
the language supplied procedures `cons', `car', and `cdr' for operating
on pairs. But the only thing we need to know about these three
operations is that if we glue two objects together using `cons' we can
retrieve the objects using `car' and `cdr'. That is, the operations
satisfy the condition that, for any objects `x' and `y', if `z' is
`(cons x y)' then `(car z)' is `x' and `(cdr z)' is `y'. Indeed, we
mentioned that these three procedures are included as primitives in our
language. However, any triple of procedures that satisfies the above
condition can be used as the basis for implementing pairs. This point
is illustrated strikingly by the fact that we could implement `cons',
`car', and `cdr' without using any data structures at all but only
using procedures. Here are the definitions:
(define (cons x y)
(define (dispatch m)
(cond ((= m 0) x)
((= m 1) y)
(else (error "Argument not 0 or 1  CONS" m))))
dispatch)
(define (car z) (z 0))
(define (cdr z) (z 1))
This use of procedures corresponds to nothing like our intuitive
notion of what data should be. Nevertheless, all we need to do to show
that this is a valid way to represent pairs is to verify that these
procedures satisfy the condition given above.
The subtle point to notice is that the value returned by `(cons x
y)' is a procedurenamely the internally defined procedure `dispatch',
which takes one argument and returns either `x' or `y' depending on
whether the argument is 0 or 1. Correspondingly, `(car z)' is defined
to apply `z' to 0. Hence, if `z' is the procedure formed by `(cons x
y)', then `z' applied to 0 will yield `x'. Thus, we have shown that
`(car (cons x y))' yields `x', as desired. Similarly, `(cdr (cons x
y))' applies the procedure returned by `(cons x y)' to 1, which returns
`y'. Therefore, this procedural implementation of pairs is a valid
implementation, and if we access pairs using only `cons', `car', and
`cdr' we cannot distinguish this implementation from one that uses
"real" data structures.
The point of exhibiting the procedural representation of pairs is
not that our language works this way (Scheme, and Lisp systems in
general, implement pairs directly, for efficiency reasons) but that it
could work this way. The procedural representation, although obscure,
is a perfectly adequate way to represent pairs, since it fulfills the
only conditions that pairs need to fulfill. This example also
demonstrates that the ability to manipulate procedures as objects
automatically provides the ability to represent compound data. This
may seem a curiosity now, but procedural representations of data will
play a central role in our programming repertoire. This style of
programming is often called "message passing", and we will be using it
as a basic tool in *Note Chapter 3:: when we address the issues of
modeling and simulation.
*Exercise 2.4:* Here is an alternative procedural representation
of pairs. For this representation, verify that `(car (cons x y))'
yields `x' for any objects `x' and `y'.
(define (cons x y)
(lambda (m) (m x y)))
(define (car z)
(z (lambda (p q) p)))
What is the corresponding definition of `cdr'? (Hint: To verify
that this works, make use of the substitution model of section
*Note 115::.)
*Exercise 2.5:* Show that we can represent pairs of nonnegative
integers using only numbers and arithmetic operations if we
represent the pair a and b as the integer that is the product 2^a
3^b. Give the corresponding definitions of the procedures `cons',
`car', and `cdr'.
*Exercise 2.6:* In case representing pairs as procedures wasn't
mindboggling enough, consider that, in a language that can
manipulate procedures, we can get by without numbers (at least
insofar as nonnegative integers are concerned) by implementing 0
and the operation of adding 1 as
(define zero (lambda (f) (lambda (x) x)))
(define (add1 n)
(lambda (f) (lambda (x) (f ((n f) x)))))
This representation is known as "Church numerals", after its
inventor, Alonzo Church, the logician who invented the [lambda]
calculus.
Define `one' and `two' directly (not in terms of `zero' and
`add1'). (Hint: Use substitution to evaluate `(add1 zero)').
Give a direct definition of the addition procedure `+' (not in
terms of repeated application of `add1').
 Footnotes 
(1) Surprisingly, this idea is very difficult to formulate
rigorously. There are two approaches to giving such a formulation. One,
pioneered by C. A. R. Hoare (1972), is known as the method of models
"abstract models". It formalizes the "procedures plus conditions"
specification as outlined in the rationalnumber example above. Note
that the condition on the rationalnumber representation was stated in
terms of facts about integers (equality and division). In general,
abstract models define new kinds of data objects in terms of previously
defined types of data objects. Assertions about data objects can
therefore be checked by reducing them to assertions about previously
defined data objects. Another approach, introduced by Zilles at MIT,
by Goguen, Thatcher, Wagner, and Wright at IBM (see Thatcher, Wagner,
and Wright 1978), and by Guttag at Toronto (see Guttag 1977), is called "algebraic
specification". It regards the "procedures" as elements of an abstract
algebraic system whose behavior is specified by axioms that correspond
to our "conditions," and uses the techniques of abstract algebra to
check assertions about data objects. Both methods are surveyed in the
paper by Liskov and Zilles (1975).
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2.1.4 Extended Exercise: Interval Arithmetic

Alyssa P. Hacker is designing a system to help people solve engineering
problems. One feature she wants to provide in her system is the
ability to manipulate inexact quantities (such as measured parameters
of physical devices) with known precision, so that when computations
are done with such approximate quantities the results will be numbers
of known precision.
Electrical engineers will be using Alyssa's system to compute
electrical quantities. It is sometimes necessary for them to compute
the value of a parallel equivalent resistance R_p of two resistors R_1
and R_2 using the formula
1
R_p = 
1/R_1 + 1/R_2
Resistance values are usually known only up to some tolerance
guaranteed by the manufacturer of the resistor. For example, if you
buy a resistor labeled "6.8 ohms with 10% tolerance" you can only be
sure that the resistor has a resistance between 6.8  0.68 = 6.12 and
6.8 + 0.68 = 7.48 ohms. Thus, if you have a 6.8ohm 10% resistor in
parallel with a 4.7ohm 5% resistor, the resistance of the combination
can range from about 2.58 ohms (if the two resistors are at the lower
bounds) to about 2.97 ohms (if the two resistors are at the upper
bounds).
Alyssa's idea is to implement "interval arithmetic" as a set of
arithmetic operations for combining "intervals" (objects that represent
the range of possible values of an inexact quantity). The result of
adding, subtracting, multiplying, or dividing two intervals is itself
an interval, representing the range of the result.
Alyssa postulates the existence of an abstract object called an
"interval" that has two endpoints: a lower bound and an upper bound.
She also presumes that, given the endpoints of an interval, she can
construct the interval using the data constructor `makeinterval'.
Alyssa first writes a procedure for adding two intervals. She reasons
that the minimum value the sum could be is the sum of the two lower
bounds and the maximum value it could be is the sum of the two upper
bounds:
(define (addinterval x y)
(makeinterval (+ (lowerbound x) (lowerbound y))
(+ (upperbound x) (upperbound y))))
Alyssa also works out the product of two intervals by finding the
minimum and the maximum of the products of the bounds and using them as
the bounds of the resulting interval. (`Min' and `max' are primitives
that find the minimum or maximum of any number of arguments.)
(define (mulinterval x y)
(let ((p1 (* (lowerbound x) (lowerbound y)))
(p2 (* (lowerbound x) (upperbound y)))
(p3 (* (upperbound x) (lowerbound y)))
(p4 (* (upperbound x) (upperbound y))))
(makeinterval (min p1 p2 p3 p4)
(max p1 p2 p3 p4))))
To divide two intervals, Alyssa multiplies the first by the
reciprocal of the second. Note that the bounds of the reciprocal
interval are the reciprocal of the upper bound and the reciprocal of
the lower bound, in that order.
(define (divinterval x y)
(mulinterval x
(makeinterval (/ 1.0 (upperbound y))
(/ 1.0 (lowerbound y)))))
*Exercise 2.7:* Alyssa's program is incomplete because she has not
specified the implementation of the interval abstraction. Here is
a definition of the interval constructor:
(define (makeinterval a b) (cons a b))
Define selectors `upperbound' and `lowerbound' to complete the
implementation.
*Exercise 2.8:* Using reasoning analogous to Alyssa's, describe
how the difference of two intervals may be computed. Define a
corresponding subtraction procedure, called `subinterval'.
*Exercise 2.9:* The "width" of an interval is half of the
difference between its upper and lower bounds. The width is a
measure of the uncertainty of the number specified by the
interval. For some arithmetic operations the width of the result
of combining two intervals is a function only of the widths of the
argument intervals, whereas for others the width of the
combination is not a function of the widths of the argument
intervals. Show that the width of the sum (or difference) of two
intervals is a function only of the widths of the intervals being
added (or subtracted). Give examples to show that this is not
true for multiplication or division.
*Exercise 2.10:* Ben Bitdiddle, an expert systems programmer,
looks over Alyssa's shoulder and comments that it is not clear what
it means to divide by an interval that spans zero. Modify
Alyssa's code to check for this condition and to signal an error
if it occurs.
*Exercise 2.11:* In passing, Ben also cryptically comments: "By
testing the signs of the endpoints of the intervals, it is
possible to break `mulinterval' into nine cases, only one of which
requires more than two multiplications." Rewrite this procedure
using Ben's suggestion.
After debugging her program, Alyssa shows it to a potential user,
who complains that her program solves the wrong problem. He wants
a program that can deal with numbers represented as a center value
and an additive tolerance; for example, he wants to work with
intervals such as 3.5 +/ 0.15 rather than [3.35, 3.65]. Alyssa
returns to her desk and fixes this problem by supplying an
alternate constructor and alternate selectors:
(define (makecenterwidth c w)
(makeinterval ( c w) (+ c w)))
(define (center i)
(/ (+ (lowerbound i) (upperbound i)) 2))
(define (width i)
(/ ( (upperbound i) (lowerbound i)) 2))
Unfortunately, most of Alyssa's users are engineers. Real
engineering situations usually involve measurements with only a
small uncertainty, measured as the ratio of the width of the
interval to the midpoint of the interval. Engineers usually
specify percentage tolerances on the parameters of devices, as in
the resistor specifications given earlier.
*Exercise 2.12:* Define a constructor `makecenterpercent' that
takes a center and a percentage tolerance and produces the desired
interval. You must also define a selector `percent' that produces
the percentage tolerance for a given interval. The `center'
selector is the same as the one shown above.
*Exercise 2.13:* Show that under the assumption of small
percentage tolerances there is a simple formula for the approximate
percentage tolerance of the product of two intervals in terms of
the tolerances of the factors. You may simplify the problem by
assuming that all numbers are positive.
After considerable work, Alyssa P. Hacker delivers her finished
system. Several years later, after she has forgotten all about
it, she gets a frenzied call from an irate user, Lem E. Tweakit.
It seems that Lem has noticed that the formula for parallel
resistors can be written in two algebraically equivalent ways:
R_1 R_2

R_1 + R_2
and
1

1/R_1 + 1/R_2
He has written the following two programs, each of which computes
the parallelresistors formula differently:
(define (par1 r1 r2)
(divinterval (mulinterval r1 r2)
(addinterval r1 r2)))
(define (par2 r1 r2)
(let ((one (makeinterval 1 1)))
(divinterval one
(addinterval (divinterval one r1)
(divinterval one r2)))))
Lem complains that Alyssa's program gives different answers for
the two ways of computing. This is a serious complaint.
*Exercise 2.14:* Demonstrate that Lem is right. Investigate the
behavior of the system on a variety of arithmetic expressions.
Make some intervals A and B, and use them in computing the
expressions A/A and A/B. You will get the most insight by using
intervals whose width is a small percentage of the center value.
Examine the results of the computation in centerpercent form (see
*Note Exercise 212::).
*Exercise 2.15:* Eva Lu Ator, another user, has also noticed the
different intervals computed by different but algebraically
equivalent expressions. She says that a formula to compute with
intervals using Alyssa's system will produce tighter error bounds
if it can be written in such a form that no variable that
represents an uncertain number is repeated. Thus, she says,
`par2' is a "better" program for parallel resistances than `par1'.
Is she right? Why?
*Exercise 2.16:* Explain, in general, why equivalent algebraic
expressions may lead to different answers. Can you devise an
intervalarithmetic package that does not have this shortcoming,
or is this task impossible? (Warning: This problem is very
difficult.)
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2.2 Hierarchical Data and the Closure Property
==============================================
As we have seen, pairs provide a primitive "glue" that we can use to
construct compound data objects. *Note Figure 22:: shows a standard
way to visualize a pairin this case, the pair formed by `(cons 1 2)'.
In this representation, which is called "boxandpointer notation",
each object is shown as a "pointer" to a box. The box for a primitive
object contains a representation of the object. For example, the box
for a number contains a numeral. The box for a pair is actually a
double box, the left part containing (a pointer to) the `car' of the
pair and the right part containing the `cdr'.
*Figure 2.2:* Boxandpointer representation of `(cons 1 2)'.
+++ ++
> *  *+> 2 
+++ ++

V
++
 1 
++
We have already seen that `cons' can be used to combine not only
numbers but pairs as well. (You made use of this fact, or should have,
in doing *Note Exercise 22:: and *Note Exercise 23::.) As a
consequence, pairs provide a universal building block from which we can
construct all sorts of data structures. *Note Figure 23:: shows two
ways to use pairs to combine the numbers 1, 2, 3, and 4.
*Figure 2.3:* Two ways to combine 1, 2, 3, and 4 using pairs.
+++ +++ +++ ++
> *  *+> *  *  > *  *+> 4 
+++ +++ +++ ++
   
V V V V
+++ ++ ++ +++ +++
 *  *   3   4   *  *+> *  * 
+++ ++ ++ +++ +++
    
V V V V V
++ ++ ++ ++ ++
 1   2   1   2   3 
++ ++ ++ ++ ++
(cons (cons 1 2) (cons (cons 1
(cons 3 4)) (cons 2 3))
4)
The ability to create pairs whose elements are pairs is the essence
of list structure's importance as a representational tool. We refer to
this ability as the "closure property" of `cons'. In general, an
operation for combining data objects satisfies the closure property if
the results of combining things with that operation can themselves be
combined using the same operation.(1) Closure is the key to power in
any means of combination because it permits us to create "hierarchical"
structuresstructures made up of parts, which themselves are made up
of parts, and so on.
From the outset of *Note Chapter 1::, we've made essential use of
closure in dealing with procedures, because all but the very simplest
programs rely on the fact that the elements of a combination can
themselves be combinations. In this section, we take up the
consequences of closure for compound data. We describe some
conventional techniques for using pairs to represent sequences and
trees, and we exhibit a graphics language that illustrates closure in a
vivid way.(2)
* Menu:
* 221:: Representing Sequences
* 222:: Hierarchical Structures
* 223:: Sequences as Conventional Interfaces
* 224:: Example: A Picture Language
 Footnotes 
(1) The use of the word "closure" here comes from abstract algebra,
where a set of elements is said to be closed under an operation if
applying the operation to elements in the set produces an element that
is again an element of the set. The Lisp community also
(unfortunately) uses the word "closure" to describe a totally unrelated
concept: A closure is an implementation technique for representing
procedures with free variables. We do not use the word "closure" in
this second sense in this book.
(2) The notion that a means of combination should satisfy closure is
a straightforward idea. Unfortunately, the data combiners provided in
many popular programming languages do not satisfy closure, or make
closure cumbersome to exploit. In Fortran or Basic, one typically
combines data elements by assembling them into arraysbut one cannot
form arrays whose elements are themselves arrays. Pascal and C admit
structures whose elements are structures. However, this requires that
the programmer manipulate pointers explicitly, and adhere to the
restriction that each field of a structure can contain only elements of
a prespecified form. Unlike Lisp with its pairs, these languages have
no builtin generalpurpose glue that makes it easy to manipulate
compound data in a uniform way. This limitation lies behind Alan
Perlis's comment in his foreword to this book: "In Pascal the plethora
of declarable data structures induces a specialization within functions
that inhibits and penalizes casual cooperation. It is better to have
100 functions operate on one data structure than to have 10 functions
operate on 10 data structures."
File: sicp.info, Node: 221, Next: 222, Prev: 22, Up: 22
2.2.1 Representing Sequences

*Figure 2.4:* The sequence 1, 2, 3, 4 represented as a chain of
pairs.
+++ +++ +++ +++
> *  *+> *  *+> *  *+> *  / 
+++ +++ +++ +++
   
V V V V
++ ++ ++ ++
 1   2   3   4 
++ ++ ++ ++
One of the useful structures we can build with pairs is a "sequence"an
ordered collection of data objects. There are, of course, many ways to
represent sequences in terms of pairs. One particularly
straightforward representation is illustrated in *Note Figure 24::,
where the sequence 1, 2, 3, 4 is represented as a chain of pairs. The
`car' of each pair is the corresponding item in the chain, and the
`cdr' of the pair is the next pair in the chain. The `cdr' of the
final pair signals the end of the sequence by pointing to a
distinguished value that is not a pair, represented in boxandpointer
diagrams as a diagonal line and in programs as the value of the
variable `nil'. The entire sequence is constructed by nested `cons'
operations:
(cons 1
(cons 2
(cons 3
(cons 4 nil))))
Such a sequence of pairs, formed by nested `cons'es, is called a "list",
and Scheme provides a primitive called `list' to help in constructing
lists.(1) The above sequence could be produced by `(list 1 2 3 4)'.
In general,
(list ... )
is equivalent to
(cons
(cons
(cons ...
(cons
nil)
...)))
Lisp systems conventionally print lists by printing the sequence of
elements, enclosed in parentheses. Thus, the data object in *Note
Figure 24:: is printed as `(1 2 3 4)':
(define onethroughfour (list 1 2 3 4))
onethroughfour
(1 2 3 4)
Be careful not to confuse the expression `(list 1 2 3 4)' with the
list `(1 2 3 4)', which is the result obtained when the expression is
evaluated. Attempting to evaluate the expression `(1 2 3 4)' will
signal an error when the interpreter tries to apply the procedure `1' to
arguments `2', `3', and `4'.
We can think of `car' as selecting the first item in the list, and of
`cdr' as selecting the sublist consisting of all but the first item.
Nested applications of `car' and `cdr' can be used to extract the
second, third, and subsequent items in the list.(2) The constructor
`cons' makes a list like the original one, but with an additional item
at the beginning.
(car onethroughfour)
1
(cdr onethroughfour)
(2 3 4)
(car (cdr onethroughfour))
2
(cons 10 onethroughfour)
(10 1 2 3 4)
(cons 5 onethroughfour)
(5 1 2 3 4)
The value of `nil', used to terminate the chain of pairs, can be
thought of as a sequence of no elements, the "empty list". The word "nil"
is a contraction of the Latin word _nihil_, which means "nothing."(3)
List operations
...............
The use of pairs to represent sequences of elements as lists is
accompanied by conventional programming techniques for manipulating
lists by successively "`cdr'ing down" the lists. For example, the
procedure `listref' takes as arguments a list and a number n and
returns the nth item of the list. It is customary to number the
elements of the list beginning with 0. The method for computing
`listref' is the following:
* For n = 0, `listref' should return the `car' of the list.
* Otherwise, `listref' should return the (n  1)st item of the
`cdr' of the list.
(define (listref items n)
(if (= n 0)
(car items)
(listref (cdr items) ( n 1))))
(define squares (list 1 4 9 16 25))
(listref squares 3)
16
Often we `cdr' down the whole list. To aid in this, Scheme includes
a primitive predicate `null?', which tests whether its argument is the
empty list. The procedure `length', which returns the number of items
in a list, illustrates this typical pattern of use:
(define (length items)
(if (null? items)
0
(+ 1 (length (cdr items)))))
(define odds (list 1 3 5 7))
(length odds)
4
The `length' procedure implements a simple recursive plan. The
reduction step is:
* The `length' of any list is 1 plus the `length' of the `cdr' of
the list.
This is applied successively until we reach the base case:
* The `length' of the empty list is 0.
We could also compute `length' in an iterative style:
(define (length items)
(define (lengthiter a count)
(if (null? a)
count
(lengthiter (cdr a) (+ 1 count))))
(lengthiter items 0))
Another conventional programming technique is to "`cons' up" an
answer list while `cdr'ing down a list, as in the procedure `append',
which takes two lists as arguments and combines their elements to make
a new list:
(append squares odds)
(1 4 9 16 25 1 3 5 7)
(append odds squares)
(1 3 5 7 1 4 9 16 25)
`Append' is also implemented using a recursive plan. To `append'
lists `list1' and `list2', do the following:
* If `list1' is the empty list, then the result is just `list2'.
* Otherwise, `append' the `cdr' of `list1' and `list2', and `cons'
the `car' of `list1' onto the result:
(define (append list1 list2)
(if (null? list1)
list2
(cons (car list1) (append (cdr list1) list2))))
*Exercise 2.17:* Define a procedure `lastpair' that returns the
list that contains only the last element of a given (nonempty)
list:
(lastpair (list 23 72 149 34))
(34)
*Exercise 2.18:* Define a procedure `reverse' that takes a list as
argument and returns a list of the same elements in reverse order:
(reverse (list 1 4 9 16 25))
(25 16 9 4 1)
*Exercise 2.19:* Consider the changecounting program of section
*Note 122::. It would be nice to be able to easily change the
currency used by the program, so that we could compute the number
of ways to change a British pound, for example. As the program is
written, the knowledge of the currency is distributed partly into
the procedure `firstdenomination' and partly into the procedure
`countchange' (which knows that there are five kinds of U.S.
coins). It would be nicer to be able to supply a list of coins to
be used for making change.
We want to rewrite the procedure `cc' so that its second argument
is a list of the values of the coins to use rather than an integer
specifying which coins to use. We could then have lists that
defined each kind of currency:
(define uscoins (list 50 25 10 5 1))
(define ukcoins (list 100 50 20 10 5 2 1 0.5))
We could then call `cc' as follows:
(cc 100 uscoins)
292
To do this will require changing the program `cc' somewhat. It
will still have the same form, but it will access its second
argument differently, as follows:
(define (cc amount coinvalues)
(cond ((= amount 0) 1)
((or (< amount 0) (nomore? coinvalues)) 0)
(else
(+ (cc amount
(exceptfirstdenomination coinvalues))
(cc ( amount
(firstdenomination coinvalues))
coinvalues)))))
Define the procedures `firstdenomination',
`exceptfirstdenomination', and `nomore?' in terms of primitive
operations on list structures. Does the order of the list
`coinvalues' affect the answer produced by `cc'? Why or why not?
*Exercise 2.20:* The procedures `+', `*', and `list' take
arbitrary numbers of arguments. One way to define such procedures
is to use `define' with notation "dottedtail notation". In a
procedure definition, a parameter list that has a dot before the
last parameter name indicates that, when the procedure is called,
the initial parameters (if any) will have as values the initial
arguments, as usual, but the final parameter's value will be a "list"
of any remaining arguments. For instance, given the definition
(define (f x y . z) )
the procedure `f' can be called with two or more arguments. If we
evaluate
(f 1 2 3 4 5 6)
then in the body of `f', `x' will be 1, `y' will be 2, and `z'
will be the list `(3 4 5 6)'. Given the definition
(define (g . w) )
the procedure `g' can be called with zero or more arguments. If we
evaluate
(g 1 2 3 4 5 6)
then in the body of `g', `w' will be the list `(1 2 3 4 5 6)'.(4)
Use this notation to write a procedure `sameparity' that takes
one or more integers and returns a list of all the arguments that
have the same evenodd parity as the first argument. For example,
(sameparity 1 2 3 4 5 6 7)
(1 3 5 7)
(sameparity 2 3 4 5 6 7)
(2 4 6)
Mapping over lists
..................
One extremely useful operation is to apply some transformation to each
element in a list and generate the list of results. For instance, the
following procedure scales each number in a list by a given factor:
(define (scalelist items factor)
(if (null? items)
nil
(cons (* (car items) factor)
(scalelist (cdr items) factor))))
(scalelist (list 1 2 3 4 5) 10)
(10 20 30 40 50)
We can abstract this general idea and capture it as a common pattern
expressed as a higherorder procedure, just as in section *Note 13::.
The higherorder procedure here is called `map'. `Map' takes as
arguments a procedure of one argument and a list, and returns a list of
the results produced by applying the procedure to each element in the
list:(5)
(define (map proc items)
(if (null? items)
nil
(cons (proc (car items))
(map proc (cdr items)))))
(map abs (list 10 2.5 11.6 17))
(10 2.5 11.6 17)
(map (lambda (x) (* x x))
(list 1 2 3 4))
(1 4 9 16)
Now we can give a new definition of `scalelist' in terms of `map':
(define (scalelist items factor)
(map (lambda (x) (* x factor))
items))
`Map' is an important construct, not only because it captures a
common pattern, but because it establishes a higher level of
abstraction in dealing with lists. In the original definition of
`scalelist', the recursive structure of the program draws attention to
the elementbyelement processing of the list. Defining `scalelist'
in terms of `map' suppresses that level of detail and emphasizes that
scaling transforms a list of elements to a list of results. The
difference between the two definitions is not that the computer is
performing a different process (it isn't) but that we think about the
process differently. In effect, `map' helps establish an abstraction
barrier that isolates the implementation of procedures that transform
lists from the details of how the elements of the list are extracted
and combined. Like the barriers shown in *Note Figure 21::, this
abstraction gives us the flexibility to change the lowlevel details of
how sequences are implemented, while preserving the conceptual
framework of operations that transform sequences to sequences. Section
*Note 223:: expands on this use of sequences as a framework for
organizing programs.
*Exercise 2.21:* The procedure `squarelist' takes a list of
numbers as argument and returns a list of the squares of those
numbers.
(squarelist (list 1 2 3 4))
(1 4 9 16)
Here are two different definitions of `squarelist'. Complete
both of them by filling in the missing expressions:
(define (squarelist items)
(if (null? items)
nil
(cons )))
(define (squarelist items)
(map ))
*Exercise 2.22:* Louis Reasoner tries to rewrite the first
`squarelist' procedure of *Note Exercise 221:: so that it
evolves an iterative process:
(define (squarelist items)
(define (iter things answer)
(if (null? things)
answer
(iter (cdr things)
(cons (square (car things))
answer))))
(iter items nil))
Unfortunately, defining `squarelist' this way produces the answer
list in the reverse order of the one desired. Why?
Louis then tries to fix his bug by interchanging the arguments to
`cons':
(define (squarelist items)
(define (iter things answer)
(if (null? things)
answer
(iter (cdr things)
(cons answer
(square (car things))))))
(iter items nil))
This doesn't work either. Explain.
*Exercise 2.23:* The procedure `foreach' is similar to `map'. It
takes as arguments a procedure and a list of elements. However,
rather than forming a list of the results, `foreach' just applies
the procedure to each of the elements in turn, from left to right.
The values returned by applying the procedure to the elements are
not used at all`foreach' is used with procedures that perform
an action, such as printing. For example,
(foreach (lambda (x) (newline) (display x))
(list 57 321 88))
57
321
88
The value returned by the call to `foreach' (not illustrated
above) can be something arbitrary, such as true. Give an
implementation of `foreach'.
 Footnotes 
(1) In this book, we use "list" to mean a chain of pairs terminated
by the endoflist marker. In contrast, the term "list structure"
refers to any data structure made out of pairs, not just to lists.
(2) Since nested applications of `car' and `cdr' are cumbersome to
write, Lisp dialects provide abbreviations for themfor instance,
(cadr (ARG)) = (car (cdr (ARG)))
The names of all such procedures start with `c' and end with `r'.
Each `a' between them stands for a `car' operation and each `d' for a
`cdr' operation, to be applied in the same order in which they appear
in the name. The names `car' and `cdr' persist because simple
combinations like `cadr' are pronounceable.
(3) It's remarkable how much energy in the standardization of Lisp
dialects has been dissipated in arguments that are literally over
nothing: Should `nil' be an ordinary name? Should the value of `nil'
be a symbol? Should it be a list? Should it be a pair? In Scheme,
`nil' is an ordinary name, which we use in this section as a variable
whose value is the endoflist marker (just as `true' is an ordinary
variable that has a true value). Other dialects of Lisp, including
Common Lisp, treat `nil' as a special symbol. The authors of this
book, who have endured too many language standardization brawls, would
like to avoid the entire issue. Once we have introduced quotation in
section *Note 23::, we will denote the empty list as `'()' and
dispense with the variable `nil' entirely.
(4) To define `f' and `g' using `lambda' we would write
(define f (lambda (x y . z) ))
(define g (lambda w ))
(5) [Footnote 12] Scheme standardly provides a `map' procedure that
is more general than the one described here. This more general `map'
takes a procedure of n arguments, together with n lists, and applies
the procedure to all the first elements of the lists, all the second
elements of the lists, and so on, returning a list of the results. For
example:
(map + (list 1 2 3) (list 40 50 60) (list 700 800 900))
(741 852 963)
(map (lambda (x y) (+ x (* 2 y)))
(list 1 2 3)
(list 4 5 6))
(9 12 15)
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2.2.2 Hierarchical Structures

The representation of sequences in terms of lists generalizes naturally
to represent sequences whose elements may themselves be sequences. For
example, we can regard the object `((1 2) 3 4)' constructed by
(cons (list 1 2) (list 3 4))
as a list of three items, the first of which is itself a list, `(1 2)'.
Indeed, this is suggested by the form in which the result is printed by
the interpreter. *Note Figure 25:: shows the representation of this
structure in terms of pairs.
*Figure 2.5:* Structure formed by `(cons (list 1 2) (list 3 4))'.
(3 4)

V
((1 2) 3 4) +++ +++ +++
> *  *+> *  *+> *  / 
+++ +++ +++
  
V V V
(1 2) +++ +++ ++ ++
> *  *+> *  /   3   4 
+++ +++ ++ ++
 
V V
++ ++
 1   2 
++ ++
Another way to think of sequences whose elements are sequences is as "trees".
The elements of the sequence are the branches of the tree, and
elements that are themselves sequences are subtrees. *Note Figure 26::
shows the structure in *Note Figure 25:: viewed as a tree.
*Figure 2.6:* The list structure in *Note Figure 25:: viewed as a
tree.
((1 2) 3 4)
/\\
/  \
(1 2) 3 4
/ \
1 2
Recursion is a natural tool for dealing with tree structures, since
we can often reduce operations on trees to operations on their
branches, which reduce in turn to operations on the branches of the
branches, and so on, until we reach the leaves of the tree. As an
example, compare the `length' procedure of section *Note 221:: with
the `countleaves' procedure, which returns the total number of leaves
of a tree:
(define x (cons (list 1 2) (list 3 4)))
(length x)
3
(countleaves x)
4
(list x x)
(((1 2) 3 4) ((1 2) 3 4))
(length (list x x))
2
(countleaves (list x x))
8
To implement `countleaves', recall the recursive plan for computing
`length':
* `Length' of a list `x' is 1 plus `length' of the `cdr' of `x'.
* `Length' of the empty list is 0.
`Countleaves' is similar. The value for the empty list is the same:
* `Countleaves' of the empty list is 0.
But in the reduction step, where we strip off the `car' of the list,
we must take into account that the `car' may itself be a tree whose
leaves we need to count. Thus, the appropriate reduction step is
* `Countleaves' of a tree `x' is `countleaves' of the `car' of `x'
plus `countleaves' of the `cdr' of `x'.
Finally, by taking `car's we reach actual leaves, so we need another
base case:
* `Countleaves' of a leaf is 1.
To aid in writing recursive procedures on trees, Scheme provides the
primitive predicate `pair?', which tests whether its argument is a
pair. Here is the complete procedure:(1)
(define (countleaves x)
(cond ((null? x) 0)
((not (pair? x)) 1)
(else (+ (countleaves (car x))
(countleaves (cdr x))))))
*Exercise 2.24:* Suppose we evaluate the expression `(list 1 (list
2 (list 3 4)))'. Give the result printed by the interpreter, the
corresponding boxandpointer structure, and the interpretation of
this as a tree (as in *Note Figure 26::).
*Exercise 2.25:* Give combinations of `car's and `cdr's that will
pick 7 from each of the following lists:
(1 3 (5 7) 9)
((7))
(1 (2 (3 (4 (5 (6 7))))))
*Exercise 2.26:* Suppose we define `x' and `y' to be two lists:
(define x (list 1 2 3))
(define y (list 4 5 6))
What result is printed by the interpreter in response to
evaluating each of the following expressions:
(append x y)
(cons x y)
(list x y)
*Exercise 2.27:* Modify your `reverse' procedure of *Note Exercise
218:: to produce a `deepreverse' procedure that takes a list as
argument and returns as its value the list with its elements
reversed and with all sublists deepreversed as well. For example,
(define x (list (list 1 2) (list 3 4)))
x
((1 2) (3 4))
(reverse x)
((3 4) (1 2))
(deepreverse x)
((4 3) (2 1))
*Exercise 2.28:* Write a procedure `fringe' that takes as argument
a tree (represented as a list) and returns a list whose elements
are all the leaves of the tree arranged in lefttoright order.
For example,
(define x (list (list 1 2) (list 3 4)))
(fringe x)
(1 2 3 4)
(fringe (list x x))
(1 2 3 4 1 2 3 4)
*Exercise 2.29:* A binary mobile consists of two branches, a left
branch and a right branch. Each branch is a rod of a certain
length, from which hangs either a weight or another binary mobile.
We can represent a binary mobile using compound data by
constructing it from two branches (for example, using `list'):
(define (makemobile left right)
(list left right))
A branch is constructed from a `length' (which must be a number)
together with a `structure', which may be either a number
(representing a simple weight) or another mobile:
(define (makebranch length structure)
(list length structure))
a. Write the corresponding selectors `leftbranch' and
`rightbranch', which return the branches of a mobile, and
`branchlength' and `branchstructure', which return the
components of a branch.
b. Using your selectors, define a procedure `totalweight' that
returns the total weight of a mobile.
c. A mobile is said to be "balanced" if the torque applied by
its topleft branch is equal to that applied by its topright
branch (that is, if the length of the left rod multiplied by
the weight hanging from that rod is equal to the
corresponding product for the right side) and if each of the
submobiles hanging off its branches is balanced. Design a
predicate that tests whether a binary mobile is balanced.
d. Suppose we change the representation of mobiles so that the
constructors are
(define (makemobile left right)
(cons left right))
(define (makebranch length structure)
(cons length structure))
How much do you need to change your programs to convert to
the new representation?
Mapping over trees
..................
Just as `map' is a powerful abstraction for dealing with sequences,
`map' together with recursion is a powerful abstraction for dealing with
trees. For instance, the `scaletree' procedure, analogous to
`scalelist' of section *Note 221::, takes as arguments a numeric
factor and a tree whose leaves are numbers. It returns a tree of the
same shape, where each number is multiplied by the factor. The
recursive plan for `scaletree' is similar to the one for
`countleaves':
(define (scaletree tree factor)
(cond ((null? tree) nil)
((not (pair? tree)) (* tree factor))
(else (cons (scaletree (car tree) factor)
(scaletree (cdr tree) factor)))))
(scaletree (list 1 (list 2 (list 3 4) 5) (list 6 7))
10)
(10 (20 (30 40) 50) (60 70))
Another way to implement `scaletree' is to regard the tree as a
sequence of subtrees and use `map'. We map over the sequence, scaling
each subtree in turn, and return the list of results. In the base
case, where the tree is a leaf, we simply multiply by the factor:
(define (scaletree tree factor)
(map (lambda (subtree)
(if (pair? subtree)
(scaletree subtree factor)
(* subtree factor)))
tree))
Many tree operations can be implemented by similar combinations of
sequence operations and recursion.
*Exercise 2.30:* Define a procedure `squaretree' analogous to the
`squarelist' procedure of *Note Exercise 221::. That is,
`squarelist' should behave as follows:
(squaretree
(list 1
(list 2 (list 3 4) 5)
(list 6 7)))
(1 (4 (9 16) 25) (36 49))
Define `squaretree' both directly (i.e., without using any
higherorder procedures) and also by using `map' and recursion.
*Exercise 2.31:* Abstract your answer to *Note Exercise 230:: to
produce a procedure `treemap' with the property that
`squaretree' could be defined as
(define (squaretree tree) (treemap square tree))
*Exercise 2.32:* We can represent a set as a list of distinct
elements, and we can represent the set of all subsets of the set as
a list of lists. For example, if the set is `(1 2 3)', then the
set of all subsets is `(() (3) (2) (2 3) (1) (1 3) (1 2) (1 2
3))'. Complete the following definition of a procedure that
generates the set of subsets of a set and give a clear explanation
of why it works:
(define (subsets s)
(if (null? s)
(list nil)
(let ((rest (subsets (cdr s))))
(append rest (map rest)))))
 Footnotes 
(1) The order of the first two clauses in the `cond' matters, since
the empty list satisfies `null?' and also is not a pair.
File: sicp.info, Node: 223, Next: 224, Prev: 222, Up: 22
2.2.3 Sequences as Conventional Interfaces

In working with compound data, we've stressed how data abstraction
permits us to design programs without becoming enmeshed in the details
of data representations, and how abstraction preserves for us the
flexibility to experiment with alternative representations. In this
section, we introduce another powerful design principle for working
with data structuresthe use of "conventional interfaces".
In section *Note 13:: we saw how program abstractions, implemented
as higherorder procedures, can capture common patterns in programs
that deal with numerical data. Our ability to formulate analogous
operations for working with compound data depends crucially on the
style in which we manipulate our data structures. Consider, for
example, the following procedure, analogous to the `countleaves'
procedure of section *Note 222::, which takes a tree as argument and
computes the sum of the squares of the leaves that are odd:
(define (sumoddsquares tree)
(cond ((null? tree) 0)
((not (pair? tree))
(if (odd? tree) (square tree) 0))
(else (+ (sumoddsquares (car tree))
(sumoddsquares (cdr tree))))))
On the surface, this procedure is very different from the following
one, which constructs a list of all the even Fibonacci numbers
_Fib_(k), where k is less than or equal to a given integer n:
(define (evenfibs n)
(define (next k)
(if (> k n)
nil
(let ((f (fib k)))
(if (even? f)
(cons f (next (+ k 1)))
(next (+ k 1))))))
(next 0))
Despite the fact that these two procedures are structurally very
different, a more abstract description of the two computations reveals
a great deal of similarity. The first program
* enumerates the leaves of a tree;
* filters them, selecting the odd ones;
* squares each of the selected ones; and
* accumulates the results using `+', starting with 0.
The second program
* enumerates the integers from 0 to n;
* computes the Fibonacci number for each integer;
* filters them, selecting the even ones; and
* accumulates the results using `cons', starting with the empty
list.
A signalprocessing engineer would find it natural to conceptualize
these processes in terms of signals flowing through a cascade of
stages, each of which implements part of the program plan, as shown in
*Note Figure 27::. In `sumoddsquares', we begin with an "enumerator",
which generates a "signal" consisting of the leaves of a given tree.
This signal is passed through a "filter", which eliminates all but the
odd elements. The resulting signal is in turn passed through a "map",
which is a "transducer" that applies the `square' procedure to each
element. The output of the map is then fed to an "accumulator", which
combines the elements using `+', starting from an initial 0. The plan
for `evenfibs' is analogous.
*Figure 2.7:* The signalflow plans for the procedures
`sumoddsquares' (top) and `evenfibs' (bottom) reveal the
commonality between the two programs.
++ ++ ++ ++
 enumerate: > filter: > map: > accumulate: 
 tree leaves   odd?   square   +, 0 
++ ++ ++ ++
++ ++ ++ ++
 enumerate: > map: > filter: > accumulate: 
 integers   fib   even?   cons, () 
++ ++ ++ ++
Unfortunately, the two procedure definitions above fail to exhibit
this signalflow structure. For instance, if we examine the
`sumoddsquares' procedure, we find that the enumeration is
implemented partly by the `null?' and `pair?' tests and partly by the
treerecursive structure of the procedure. Similarly, the accumulation
is found partly in the tests and partly in the addition used in the
recursion. In general, there are no distinct parts of either procedure
that correspond to the elements in the signalflow description. Our
two procedures decompose the computations in a different way, spreading
the enumeration over the program and mingling it with the map, the
filter, and the accumulation. If we could organize our programs to
make the signalflow structure manifest in the procedures we write, this
would increase the conceptual clarity of the resulting code.
Sequence Operations
...................
The key to organizing programs so as to more clearly reflect the
signalflow structure is to concentrate on the "signals" that flow from
one stage in the process to the next. If we represent these signals as
lists, then we can use list operations to implement the processing at
each of the stages. For instance, we can implement the mapping stages
of the signalflow diagrams using the `map' procedure from section
*Note 221:::
(map square (list 1 2 3 4 5))
(1 4 9 16 25)
Filtering a sequence to select only those elements that satisfy a
given predicate is accomplished by
(define (filter predicate sequence)
(cond ((null? sequence) nil)
((predicate (car sequence))
(cons (car sequence)
(filter predicate (cdr sequence))))
(else (filter predicate (cdr sequence)))))
For example,
(filter odd? (list 1 2 3 4 5))
(1 3 5)
Accumulations can be implemented by
(define (accumulate op initial sequence)
(if (null? sequence)
initial
(op (car sequence)
(accumulate op initial (cdr sequence)))))
(accumulate + 0 (list 1 2 3 4 5))
15
(accumulate * 1 (list 1 2 3 4 5))
120
(accumulate cons nil (list 1 2 3 4 5))
(1 2 3 4 5)
All that remains to implement signalflow diagrams is to enumerate
the sequence of elements to be processed. For `evenfibs', we need to
generate the sequence of integers in a given range, which we can do as
follows:
(define (enumerateinterval low high)
(if (> low high)
nil
(cons low (enumerateinterval (+ low 1) high))))
(enumerateinterval 2 7)
(2 3 4 5 6 7)
To enumerate the leaves of a tree, we can use(1)
(define (enumeratetree tree)
(cond ((null? tree) nil)
((not (pair? tree)) (list tree))
(else (append (enumeratetree (car tree))
(enumeratetree (cdr tree))))))
(enumeratetree (list 1 (list 2 (list 3 4)) 5))
(1 2 3 4 5)
Now we can reformulate `sumoddsquares' and `evenfibs' as in the
signalflow diagrams. For `sumoddsquares', we enumerate the sequence
of leaves of the tree, filter this to keep only the odd numbers in the
sequence, square each element, and sum the results:
(define (sumoddsquares tree)
(accumulate +
0
(map square
(filter odd?
(enumeratetree tree)))))
For `evenfibs', we enumerate the integers from 0 to n, generate the
Fibonacci number for each of these integers, filter the resulting
sequence to keep only the even elements, and accumulate the results
into a list:
(define (evenfibs n)
(accumulate cons
nil
(filter even?
(map fib
(enumerateinterval 0 n)))))
The value of expressing programs as sequence operations is that this
helps us make program designs that are modular, that is, designs that
are constructed by combining relatively independent pieces. We can
encourage modular design by providing a library of standard components
together with a conventional interface for connecting the components in
flexible ways.
Modular construction is a powerful strategy for controlling
complexity in engineering design. In real signalprocessing
applications, for example, designers regularly build systems by
cascading elements selected from standardized families of filters and
transducers. Similarly, sequence operations provide a library of
standard program elements that we can mix and match. For instance, we
can reuse pieces from the `sumoddsquares' and `evenfibs' procedures
in a program that constructs a list of the squares of the first n + 1
Fibonacci numbers:
(define (listfibsquares n)
(accumulate cons
nil
(map square
(map fib
(enumerateinterval 0 n)))))
(listfibsquares 10)
(0 1 1 4 9 25 64 169 441 1156 3025)
We can rearrange the pieces and use them in computing the product of
the odd integers in a sequence:
(define (productofsquaresofoddelements sequence)
(accumulate *
1
(map square
(filter odd? sequence))))
(productofsquaresofoddelements (list 1 2 3 4 5))
225
We can also formulate conventional dataprocessing applications in
terms of sequence operations. Suppose we have a sequence of personnel
records and we want to find the salary of the highestpaid programmer.
Assume that we have a selector `salary' that returns the salary of a
record, and a predicate `programmer?' that tests if a record is for a
programmer. Then we can write
(define (salaryofhighestpaidprogrammer records)
(accumulate max
0
(map salary
(filter programmer? records))))
These examples give just a hint of the vast range of operations that
can be expressed as sequence operations.(2)
Sequences, implemented here as lists, serve as a conventional
interface that permits us to combine processing modules. Additionally,
when we uniformly represent structures as sequences, we have localized
the datastructure dependencies in our programs to a small number of
sequence operations. By changing these, we can experiment with
alternative representations of sequences, while leaving the overall
design of our programs intact. We will exploit this capability in
section *Note 35::, when we generalize the sequenceprocessing
paradigm to admit infinite sequences.
*Exercise 2.33:* Fill in the missing expressions to complete the
following definitions of some basic listmanipulation operations
as accumulations:
(define (map p sequence)
(accumulate (lambda (x y) ) nil sequence))
(define (append seq1 seq2)
(accumulate cons ))
(define (length sequence)
(accumulate 0 sequence))
*Exercise 2.34:* Evaluating a polynomial in x at a given value of
x can be formulated as an accumulation. We evaluate the polynomial
a_n r^n  a_(n1) r^(n1) + ... + a_1 r + a_0
using a wellknown algorithm called "Horner's rule", which
structures the computation as
(... (a_n r + a_(n1)) r + ... + a_1) r + a_0
In other words, we start with a_n, multiply by x, add a_(n1),
multiply by x, and so on, until we reach a_0.(3)
Fill in the following template to produce a procedure that
evaluates a polynomial using Horner's rule. Assume that the
coefficients of the polynomial are arranged in a sequence, from
a_0 through a_n.
(define (hornereval x coefficientsequence)
(accumulate (lambda (thiscoeff higherterms) )
0
coefficientsequence))
For example, to compute 1 + 3x + 5x^3 + x^(5) at x = 2 you would
evaluate
(hornereval 2 (list 1 3 0 5 0 1))
*Exercise 2.35:* Redefine `countleaves' from section *Note
222:: as an accumulation:
(define (countleaves t)
(accumulate (map )))
*Exercise 2.36:* The procedure `accumulaten' is similar to
`accumulate' except that it takes as its third argument a sequence
of sequences, which are all assumed to have the same number of
elements. It applies the designated accumulation procedure to
combine all the first elements of the sequences, all the second
elements of the sequences, and so on, and returns a sequence of
the results. For instance, if `s' is a sequence containing four
sequences, `((1 2 3) (4 5 6) (7 8 9) (10 11 12)),' then the value
of `(accumulaten + 0 s)' should be the sequence `(22 26 30)'.
Fill in the missing expressions in the following definition of
`accumulaten':
(define (accumulaten op init seqs)
(if (null? (car seqs))
nil
(cons (accumulate op init )
(accumulaten op init ))))
Exercise 2.37
.............
Suppose we represent vectors v = (v_i) as sequences of numbers, and
matrices m = (m_(ij)) as sequences of vectors (the rows of the matrix).
For example, the matrix
+ +
 1 2 3 4 
 4 5 6 6 
 6 7 8 9 
+ +
is represented as the sequence `((1 2 3 4) (4 5 6 6) (6 7 8 9))'. With
this representation, we can use sequence operations to concisely
express the basic matrix and vector operations. These operations
(which are described in any book on matrix algebra) are the following:
__
(dotproduct v w) returns the sum >_i v_i w_i
(matrix*vector m v) returns the vector t,
__
where t_i = >_j m_(ij) v_j
(matrix*matrix m n) returns the matrix p,
__
where p_(ij) = >_k m_(ik) n_(kj)
(transpose m) returns the matrix n,
where n_(ij) = m_(ji)
We can define the dot product as(4)
(define (dotproduct v w)
(accumulate + 0 (map * v w)))
Fill in the missing expressions in the following procedures for
computing the other matrix operations. (The procedure `accumulaten'
is defined in *Note Exercise 236::.)
(define (matrix*vector m v)
(map m))
(define (transpose mat)
(accumulaten mat))
(define (matrix*matrix m n)
(let ((cols (transpose n)))
(map m)))
*Exercise 2.38:* The `accumulate' procedure is also known as
`foldright', because it combines the first element of the
sequence with the result of combining all the elements to the
right. There is also a `foldleft', which is similar to
`foldright', except that it combines elements working in the
opposite direction:
(define (foldleft op initial sequence)
(define (iter result rest)
(if (null? rest)
result
(iter (op result (car rest))
(cdr rest))))
(iter initial sequence))
What are the values of
(foldright / 1 (list 1 2 3))
(foldleft / 1 (list 1 2 3))
(foldright list nil (list 1 2 3))
(foldleft list nil (list 1 2 3))
Give a property that `op' should satisfy to guarantee that
`foldright' and `foldleft' will produce the same values for any
sequence.
*Exercise 2.39:* Complete the following definitions of `reverse'
(*Note Exercise 218::) in terms of `foldright' and `foldleft'
from *Note Exercise 238:::
(define (reverse sequence)
(foldright (lambda (x y) ) nil sequence))
(define (reverse sequence)
(foldleft (lambda (x y) ) nil sequence))
Nested Mappings
...............
We can extend the sequence paradigm to include many computations that
are commonly expressed using nested loops.(5) Consider this problem:
Given a positive integer n, find all ordered pairs of distinct positive
integers i and j, where 1 <= j< i<= n, such that i + j is prime. For
example, if n is 6, then the pairs are the following:
i  2 3 4 4 5 6 6
j  1 2 1 3 2 1 5
+
i + j  3 5 5 7 7 7 11
A natural way to organize this computation is to generate the
sequence of all ordered pairs of positive integers less than or equal
to n, filter to select those pairs whose sum is prime, and then, for
each pair (i, j) that passes through the filter, produce the triple
(i,j,i + j).
Here is a way to generate the sequence of pairs: For each integer i
<= n, enumerate the integers jpainter segmentlist)
(lambda (frame)
(foreach
(lambda (segment)
(drawline
((framecoordmap frame) (startsegment segment))
((framecoordmap frame) (endsegment segment))))
segmentlist)))
The segments are given using coordinates with respect to the unit
square. For each segment in the list, the painter transforms the
segment endpoints with the frame coordinate map and draws a line
between the transformed points.
Representing painters as procedures erects a powerful abstraction
barrier in the picture language. We can create and intermix all sorts
of primitive painters, based on a variety of graphics capabilities. The
details of their implementation do not matter. Any procedure can serve
as a painter, provided that it takes a frame as argument and draws
something scaled to fit the frame.(7)
*Exercise 2.48:* A directed line segment in the plane can be
represented as a pair of vectorsthe vector running from the
origin to the startpoint of the segment, and the vector running
from the origin to the endpoint of the segment. Use your vector
representation from *Note Exercise 246:: to define a
representation for segments with a constructor `makesegment' and
selectors `startsegment' and `endsegment'.
*Exercise 2.49:* Use `segments>painter' to define the following
primitive painters:
a. The painter that draws the outline of the designated frame.
b. The painter that draws an "X" by connecting opposite corners
of the frame.
c. The painter that draws a diamond shape by connecting the
midpoints of the sides of the frame.
d. The `wave' painter.
Transforming and combining painters
...................................
An operation on painters (such as `flipvert' or `beside') works by
creating a painter that invokes the original painters with respect to
frames derived from the argument frame. Thus, for example, `flipvert'
doesn't have to know how a painter works in order to flip itit just
has to know how to turn a frame upside down: The flipped painter just
uses the original painter, but in the inverted frame.
Painter operations are based on the procedure `transformpainter',
which takes as arguments a painter and information on how to transform
a frame and produces a new painter. The transformed painter, when
called on a frame, transforms the frame and calls the original painter
on the transformed frame. The arguments to `transformpainter' are
points (represented as vectors) that specify the corners of the new
frame: When mapped into the frame, the first point specifies the new
frame's origin and the other two specify the ends of its edge vectors.
Thus, arguments within the unit square specify a frame contained within
the original frame.
(define (transformpainter painter origin corner1 corner2)
(lambda (frame)
(let ((m (framecoordmap frame)))
(let ((neworigin (m origin)))
(painter
(makeframe neworigin
(subvect (m corner1) neworigin)
(subvect (m corner2) neworigin)))))))
Here's how to flip painter images vertically:
(define (flipvert painter)
(transformpainter painter
(makevect 0.0 1.0) ; new `origin'
(makevect 1.0 1.0) ; new end of `edge1'
(makevect 0.0 0.0))) ; new end of `edge2'
Using `transformpainter', we can easily define new transformations.
For example, we can define a painter that shrinks its image to the
upperright quarter of the frame it is given:
(define (shrinktoupperright painter)
(transformpainter painter
(makevect 0.5 0.5)
(makevect 1.0 0.5)
(makevect 0.5 1.0)))
Other transformations rotate images counterclockwise by 90 degrees(8)
(define (rotate90 painter)
(transformpainter painter
(makevect 1.0 0.0)
(makevect 1.0 1.0)
(makevect 0.0 0.0)))
or squash images towards the center of the frame:(9)
(define (squashinwards painter)
(transformpainter painter
(makevect 0.0 0.0)
(makevect 0.65 0.35)
(makevect 0.35 0.65)))
Frame transformation is also the key to defining means of combining
two or more painters. The `beside' procedure, for example, takes two
painters, transforms them to paint in the left and right halves of an
argument frame respectively, and produces a new, compound painter.
When the compound painter is given a frame, it calls the first
transformed painter to paint in the left half of the frame and calls
the second transformed painter to paint in the right half of the frame:
(define (beside painter1 painter2)
(let ((splitpoint (makevect 0.5 0.0)))
(let ((paintleft
(transformpainter painter1
(makevect 0.0 0.0)
splitpoint
(makevect 0.0 1.0)))
(paintright
(transformpainter painter2
splitpoint
(makevect 1.0 0.0)
(makevect 0.5 1.0))))
(lambda (frame)
(paintleft frame)
(paintright frame)))))
Observe how the painter data abstraction, and in particular the
representation of painters as procedures, makes `beside' easy to
implement. The `beside' procedure need not know anything about the
details of the component painters other than that each painter will
draw something in its designated frame.
*Exercise 2.50:* Define the transformation `fliphoriz', which
flips painters horizontally, and transformations that rotate
painters counterclockwise by 180 degrees and 270 degrees.
*Exercise 2.51:* Define the `below' operation for painters.
`Below' takes two painters as arguments. The resulting painter,
given a frame, draws with the first painter in the bottom of the
frame and with the second painter in the top. Define `below' in
two different waysfirst by writing a procedure that is analogous
to the `beside' procedure given above, and again in terms of
`beside' and suitable rotation operations (from *Note Exercise
250::).
Levels of language for robust design
....................................
The picture language exercises some of the critical ideas we've
introduced about abstraction with procedures and data. The fundamental
data abstractions, painters, are implemented using procedural
representations, which enables the language to handle different basic
drawing capabilities in a uniform way. The means of combination
satisfy the closure property, which permits us to easily build up
complex designs. Finally, all the tools for abstracting procedures are
available to us for abstracting means of combination for painters.
We have also obtained a glimpse of another crucial idea about
languages and program design. This is the approach of "stratified
design", the notion that a complex system should be structured as a
sequence of levels that are described using a sequence of languages.
Each level is constructed by combining parts that are regarded as
primitive at that level, and the parts constructed at each level are
used as primitives at the next level. The language used at each level
of a stratified design has primitives, means of combination, and means
of abstraction appropriate to that level of detail.
Stratified design pervades the engineering of complex systems. For
example, in computer engineering, resistors and transistors are
combined (and described using a language of analog circuits) to produce
parts such as andgates and orgates, which form the primitives of a
language for digitalcircuit design.(10) These parts are combined to
build processors, bus structures, and memory systems, which are in turn
combined to form computers, using languages appropriate to computer
architecture. Computers are combined to form distributed systems, using
languages appropriate for describing network interconnections, and so
on.
As a tiny example of stratification, our picture language uses
primitive elements (primitive painters) that are created using a
language that specifies points and lines to provide the lists of line
segments for `segments>painter', or the shading details for a painter
like `rogers'. The bulk of our description of the picture language
focused on combining these primitives, using geometric combiners such
as `beside' and `below'. We also worked at a higher level, regarding
`beside' and `below' as primitives to be manipulated in a language
whose operations, such as `squareoffour', capture common patterns of
combining geometric combiners.
Stratified design helps make programs "robust", that is, it makes it
likely that small changes in a specification will require
correspondingly small changes in the program. For instance, suppose we
wanted to change the image based on `wave' shown in *Note Figure 29::.
We could work at the lowest level to change the detailed appearance of
the `wave' element; we could work at the middle level to change the way
`cornersplit' replicates the `wave'; we could work at the highest
level to change how `squarelimit' arranges the four copies of the
corner. In general, each level of a stratified design provides a
different vocabulary for expressing the characteristics of the system,
and a different kind of ability to change it.
*Exercise 2.52:* Make changes to the square limit of `wave' shown
in *Note Figure 29:: by working at each of the levels described
above. In particular:
a. Add some segments to the primitive `wave' painter of *Note
Exercise 249:: (to add a smile, for example).
b. Change the pattern constructed by `cornersplit' (for
example, by using only one copy of the `upsplit' and
`rightsplit' images instead of two).
c. Modify the version of `squarelimit' that uses
`squareoffour' so as to assemble the corners in a different
pattern. (For example, you might make the big Mr. Rogers
look outward from each corner of the square.)
 Footnotes 
(1) The picture language is based on the language Peter Henderson
created to construct images like M.C. Escher's "Square Limit" woodcut
(see Henderson 1982). The woodcut incorporates a repeated scaled
pattern, similar to the arrangements drawn using the `squarelimit'
procedure in this section.
(2) William Barton Rogers (18041882) was the founder and first
president of MIT. A geologist and talented teacher, he taught at
William and Mary College and at the University of Virginia. In 1859 he
moved to Boston, where he had more time for research, worked on a plan
for establishing a "polytechnic institute," and served as
Massachusetts's first State Inspector of Gas Meters.
When MIT was established in 1861, Rogers was elected its first
president. Rogers espoused an ideal of "useful learning" that was
different from the university education of the time, with its
overemphasis on the classics, which, as he wrote, "stand in the way of
the broader, higher and more practical instruction and discipline of
the natural and social sciences." This education was likewise to be
different from narrow tradeschool education. In Rogers's words:
The worldenforced distinction between the practical and the
scientific worker is utterly futile, and the whole experience of
modern times has demonstrated its utter worthlessness.
Rogers served as president of MIT until 1870, when he resigned due to
ill health. In 1878 the second president of MIT, John Runkle, resigned
under the pressure of a financial crisis brought on by the Panic of
1873 and strain of fighting off attempts by Harvard to take over MIT.
Rogers returned to hold the office of president until 1881.
Rogers collapsed and died while addressing MIT's graduating class at
the commencement exercises of 1882. Runkle quoted Rogers's last words
in a memorial address delivered that same year:
"As I stand here today and see what the Institute is, ... I call
to mind the beginnings of science. I remember one hundred and
fifty years ago Stephen Hales published a pamphlet on the subject
of illuminating gas, in which he stated that his researches had
demonstrated that 128 grains of bituminous coal  " "Bituminous
coal," these were his last words on earth. Here he bent forward,
as if consulting some notes on the table before him, then slowly
regaining an erect position, threw up his hands, and was
translated from the scene of his earthly labors and triumphs to
"the tomorrow of death," where the mysteries of life are solved,
and the disembodied spirit finds unending satisfaction in
contemplating the new and still unfathomable mysteries of the
infinite future.
In the words of Francis A. Walker (MIT's third president):
All his life he had borne himself most faithfully and heroically,
and he died as so good a knight would surely have wished, in
harness, at his post, and in the very part and act of public duty.
(3) Equivalently, we could write
(define flippedpairs
(squareoffour identity flipvert identity flipvert))
(4) `Rotate180' rotates a painter by 180 degrees (see *Note Exercise
250::). Instead of `rotate180' we could say `(compose flipvert
fliphoriz)', using the `compose' procedure from *Note Exercise 142::.
(5) `Framecoordmap' uses the vector operations described in *Note
Exercise 246:: below, which we assume have been implemented using some
representation for vectors. Because of data abstraction, it doesn't
matter what this vector representation is, so long as the vector
operations behave correctly.
(6) `Segments>painter' uses the representation for line segments
described in *Note Exercise 248:: below. It also uses the `foreach'
procedure described in *Note Exercise 223::.
(7) For example, the `rogers' painter of *Note Figure 211:: was
constructed from a graylevel image. For each point in a given frame,
the `rogers' painter determines the point in the image that is mapped
to it under the frame coordinate map, and shades it accordingly. By
allowing different types of painters, we are capitalizing on the
abstract data idea discussed in section *Note 213::, where we argued
that a rationalnumber representation could be anything at all that
satisfies an appropriate condition. Here we're using the fact that a
painter can be implemented in any way at all, so long as it draws
something in the designated frame. Section *Note 213:: also showed
how pairs could be implemented as procedures. Painters are our second
example of a procedural representation for data.
(8) `Rotate90' is a pure rotation only for square frames, because it
also stretches and shrinks the image to fit into the rotated frame.
(9) The diamondshaped images in figures *Note Figure 210:: and
*Note Figure 211:: were created with `squashinwards' applied to
`wave' and `rogers'.
(10) Section *Note 334:: describes one such language.
File: sicp.info, Node: 23, Next: 24, Prev: 22, Up: Chapter 2
2.3 Symbolic Data
=================
All the compound data objects we have used so far were constructed
ultimately from numbers. In this section we extend the
representational capability of our language by introducing the ability
to work with arbitrary symbols as data.
* Menu:
* 231:: Quotation
* 232:: Example: Symbolic Differentiation
* 233:: Example: Representing Sets
* 234:: Example: Huffman Encoding Trees
File: sicp.info, Node: 231, Next: 232, Prev: 23, Up: 23
2.3.1 Quotation

If we can form compound data using symbols, we can have lists such as
(a b c d)
(23 45 17)
((Norah 12) (Molly 9) (Anna 7) (Lauren 6) (Charlotte 4))
Lists containing symbols can look just like the expressions of our
language:
(* (+ 23 45) (+ x 9))
(define (fact n) (if (= n 1) 1 (* n (fact ( n 1)))))
In order to manipulate symbols we need a new element in our
language: the ability to "quote" a data object. Suppose we want to
construct the list `(a b)'. We can't accomplish this with `(list a
b)', because this expression constructs a list of the "values" of `a'
and `b' rather than the symbols themselves. This issue is well known
in the context of natural languages, where words and sentences may be
regarded either as semantic entities or as character strings (syntactic
entities). The common practice in natural languages is to use
quotation marks to indicate that a word or a sentence is to be treated
literally as a string of characters. For instance, the first letter of
"John" is clearly "J." If we tell somebody "say your name aloud," we
expect to hear that person's name. However, if we tell somebody "say
`your name' aloud," we expect to hear the words "your name." Note that
we are forced to nest quotation marks to describe what somebody else
might say.(1)
We can follow this same practice to identify lists and symbols that
are to be treated as data objects rather than as expressions to be
evaluated. However, our format for quoting differs from that of
natural languages in that we place a quotation mark (traditionally, the
single quote symbol `'') only at the beginning of the object to be
quoted. We can get away with this in Scheme syntax because we rely on
blanks and parentheses to delimit objects. Thus, the meaning of the
single quote character is to quote the next object.(2)
Now we can distinguish between symbols and their values:
(define a 1)
(define b 2)
(list a b)
(1 2)
(list 'a 'b)
(a b)
(list 'a b)
(a 2)
Quotation also allows us to type in compound objects, using the
conventional printed representation for lists:(3)
(car '(a b c))
a
(cdr '(a b c))
(b c)
In keeping with this, we can obtain the empty list by evaluating
`'()', and thus dispense with the variable `nil'.
One additional primitive used in manipulating symbols is `eq?', which
takes two symbols as arguments and tests whether they are the same.(4)
Using `eq?', we can implement a useful procedure called `memq'. This
takes two arguments, a symbol and a list. If the symbol is not
contained in the list (i.e., is not `eq?' to any item in the list),
then `memq' returns false. Otherwise, it returns the sublist of the
list beginning with the first occurrence of the symbol:
(define (memq item x)
(cond ((null? x) false)
((eq? item (car x)) x)
(else (memq item (cdr x)))))
For example, the value of
(memq 'apple '(pear banana prune))
is false, whereas the value of
(memq 'apple '(x (apple sauce) y apple pear))
is `(apple pear)'.
*Exercise 2.53:* What would the interpreter print in response to
evaluating each of the following expressions?
(list 'a 'b 'c)
(list (list 'george))
(cdr '((x1 x2) (y1 y2)))
(cadr '((x1 x2) (y1 y2)))
(pair? (car '(a short list)))
(memq 'red '((red shoes) (blue socks)))
(memq 'red '(red shoes blue socks))
*Exercise 2.54:* Two lists are said to be `equal?' if they contain
equal elements arranged in the same order. For example,
(equal? '(this is a list) '(this is a list))
is true, but
(equal? '(this is a list) '(this (is a) list))
is false. To be more precise, we can define `equal?' recursively
in terms of the basic `eq?' equality of symbols by saying that `a'
and `b' are `equal?' if they are both symbols and the symbols are
`eq?', or if they are both lists such that `(car a)' is `equal?'
to `(car b)' and `(cdr a)' is `equal?' to `(cdr b)'. Using this
idea, implement `equal?' as a procedure.(5)
*Exercise 2.55:* Eva Lu Ator types to the interpreter the
expression
(car ''abracadabra)
To her surprise, the interpreter prints back `quote'. Explain.
 Footnotes 
(1) Allowing quotation in a language wreaks havoc with the ability
to reason about the language in simple terms, because it destroys the
notion that equals can be substituted for equals. For example, three
is one plus two, but the word "three" is not the phrase "one plus two."
Quotation is powerful because it gives us a way to build expressions
that manipulate other expressions (as we will see when we write an
interpreter in *Note Chapter 4::). But allowing statements in a
language that talk about other statements in that language makes it
very difficult to maintain any coherent principle of what "equals can
be substituted for equals" should mean. For example, if we know that
the evening star is the morning star, then from the statement "the
evening star is Venus" we can deduce "the morning star is Venus."
However, given that "John knows that the evening star is Venus" we
cannot infer that "John knows that the morning star is Venus."
(2) The single quote is different from the double quote we have been
using to enclose character strings to be printed. Whereas the single
quote can be used to denote lists or symbols, the double quote is used
only with character strings. In this book, the only use for character
strings is as items to be printed.
(3) Strictly, our use of the quotation mark violates the general
rule that all compound expressions in our language should be delimited
by parentheses and look like lists. We can recover this consistency by
introducing a special form `quote', which serves the same purpose as
the quotation mark. Thus, we would type `(quote a)' instead of `'a',
and we would type `(quote (a b c))' instead of `'(a b c)'. This is
precisely how the interpreter works. The quotation mark is just a
singlecharacter abbreviation for wrapping the next complete expression
with `quote' to form `(quote )'. This is important because
it maintains the principle that any expression seen by the interpreter
can be manipulated as a data object. For instance, we could construct
the expression `(car '(a b c))', which is the same as `(car (quote (a b
c)))', by evaluating `(list 'car (list 'quote '(a b c)))'.
(4) We can consider two symbols to be "the same" if they consist of
the same characters in the same order. Such a definition skirts a deep
issue that we are not yet ready to address: the meaning of "sameness"
in a programming language. We will return to this in *Note Chapter 3::
(section *Note 313::).
(5) In practice, programmers use `equal?' to compare lists that
contain numbers as well as symbols. Numbers are not considered to be
symbols. The question of whether two numerically equal numbers (as
tested by `=') are also `eq?' is highly implementationdependent. A
better definition of `equal?' (such as the one that comes as a
primitive in Scheme) would also stipulate that if `a' and `b' are both
numbers, then `a' and `b' are `equal?' if they are numerically equal.
File: sicp.info, Node: 232, Next: 233, Prev: 231, Up: 23
2.3.2 Example: Symbolic Differentiation

As an illustration of symbol manipulation and a further illustration of
data abstraction, consider the design of a procedure that performs
symbolic differentiation of algebraic expressions. We would like the
procedure to take as arguments an algebraic expression and a variable
and to return the derivative of the expression with respect to the
variable. For example, if the arguments to the procedure are ax^2 + bx
+ c and x, the procedure should return 2ax + b. Symbolic
differentiation is of special historical significance in Lisp. It was
one of the motivating examples behind the development of a computer
language for symbol manipulation. Furthermore, it marked the beginning
of the line of research that led to the development of powerful systems
for symbolic mathematical work, which are currently being used by a
growing number of applied mathematicians and physicists.
In developing the symbolicdifferentiation program, we will follow
the same strategy of data abstraction that we followed in developing
the rationalnumber system of section *Note 211::. That is, we will
first define a differentiation algorithm that operates on abstract
objects such as "sums," "products," and "variables" without worrying
about how these are to be represented. Only afterward will we address
the representation problem.
The differentiation program with abstract data
..............................................
In order to keep things simple, we will consider a very simple
symbolicdifferentiation program that handles expressions that are
built up using only the operations of addition and multiplication with
two arguments. Differentiation of any such expression can be carried
out by applying the following reduction rules:
dc
 = 0 for c a constant, or a variable different from x
dx
dx
 = 1
dx
d(u + v) du dv
 =  + 
dx dx dx
d(uv) / dv \ / du \
 = u    + v   
dx \ dx / \ dx /
Observe that the latter two rules are recursive in nature. That is,
to obtain the derivative of a sum we first find the derivatives of the
terms and add them. Each of the terms may in turn be an expression
that needs to be decomposed. Decomposing into smaller and smaller
pieces will eventually produce pieces that are either constants or
variables, whose derivatives will be either 0 or 1.
To embody these rules in a procedure we indulge in a little wishful
thinking, as we did in designing the rationalnumber implementation.
If we had a means for representing algebraic expressions, we should be
able to tell whether an expression is a sum, a product, a constant, or
a variable. We should be able to extract the parts of an expression.
For a sum, for example we want to be able to extract the addend (first
term) and the augend (second term). We should also be able to
construct expressions from parts. Let us assume that we already have
procedures to implement the following selectors, constructors, and
predicates:
(variable? e) Is `e' a variable?
(samevariable? v1 v2) Are `v1' and `v2' the same variable?
(sum? e) Is `e' a sum?
(addend e) Addend of the sum `e'.
(augend e) Augend of the sum `e'.
(makesum a1 a2) Construct the sum of `a1' and `a2'.
(product? e) Is `e' a product?
(multiplier e) Multiplier of the product `e'.
(multiplicand e) Multiplicand of the product `e'.
(makeproduct m1 m2) Construct the product of `m1' and `m2'.
Using these, and the primitive predicate `number?', which identifies
numbers, we can express the differentiation rules as the following
procedure:
(define (deriv exp var)
(cond ((number? exp) 0)
((variable? exp)
(if (samevariable? exp var) 1 0))
((sum? exp)
(makesum (deriv (addend exp) var)
(deriv (augend exp) var)))
((product? exp)
(makesum
(makeproduct (multiplier exp)
(deriv (multiplicand exp) var))
(makeproduct (deriv (multiplier exp) var)
(multiplicand exp))))
(else
(error "unknown expression type  DERIV" exp))))
This `deriv' procedure incorporates the complete differentiation
algorithm. Since it is expressed in terms of abstract data, it will
work no matter how we choose to represent algebraic expressions, as
long as we design a proper set of selectors and constructors. This is
the issue we must address next.
Representing algebraic expressions
..................................
We can imagine many ways to use list structure to represent algebraic
expressions. For example, we could use lists of symbols that mirror
the usual algebraic notation, representing ax + b as the list `(a * x +
b)' . However, one especially straightforward choice is to use the same
parenthesized prefix notation that Lisp uses for combinations; that is,
to represent ax + b as `(+ (* a x) b)'. Then our data representation
for the differentiation problem is as follows:
* The variables are symbols. They are identified by the primitive
predicate `symbol?':
(define (variable? x) (symbol? x))
* Two variables are the same if the symbols representing them are
`eq?':
(define (samevariable? v1 v2)
(and (variable? v1) (variable? v2) (eq? v1 v2)))
* Sums and products are constructed as lists:
(define (makesum a1 a2) (list '+ a1 a2))
(define (makeproduct m1 m2) (list '* m1 m2))
* A sum is a list whose first element is the symbol `+':
(define (sum? x)
(and (pair? x) (eq? (car x) '+)))
* The addend is the second item of the sum list:
(define (addend s) (cadr s))
* The augend is the third item of the sum list:
(define (augend s) (caddr s))
* A product is a list whose first element is the symbol `*':
(define (product? x)
(and (pair? x) (eq? (car x) '*)))
* The multiplier is the second item of the product list:
(define (multiplier p) (cadr p))
* The multiplicand is the third item of the product list:
(define (multiplicand p) (caddr p))
Thus, we need only combine these with the algorithm as embodied by
`deriv' in order to have a working symbolicdifferentiation program.
Let us look at some examples of its behavior:
(deriv '(+ x 3) 'x)
(+ 1 0)
(deriv '(* x y) 'x)
(+ (* x 0) (* 1 y))
(deriv '(* (* x y) (+ x 3)) 'x)
(+ (* (* x y) (+ 1 0))
(* (+ (* x 0) (* 1 y))
(+ x 3)))
The program produces answers that are correct; however, they are
unsimplified. It is true that
d(xy)
 = x * 0 + 1 * y
dx
but we would like the program to know that x * 0 = 0, 1 * y = y, and 0
+ y = y. The answer for the second example should have been simply
`y'. As the third example shows, this becomes a serious issue when the
expressions are complex.
Our difficulty is much like the one we encountered with the
rationalnumber implementation: we haven't reduced answers to simplest
form. To accomplish the rationalnumber reduction, we needed to change
only the constructors and the selectors of the implementation. We can
adopt a similar strategy here. We won't change `deriv' at all.
Instead, we will change `makesum' so that if both summands are
numbers, `makesum' will add them and return their sum. Also, if one
of the summands is 0, then `makesum' will return the other summand.
(define (makesum a1 a2)
(cond ((=number? a1 0) a2)
((=number? a2 0) a1)
((and (number? a1) (number? a2)) (+ a1 a2))
(else (list '+ a1 a2))))
This uses the procedure `=number?', which checks whether an
expression is equal to a given number:
(define (=number? exp num)
(and (number? exp) (= exp num)))
Similarly, we will change `makeproduct' to build in the rules that 0
times anything is 0 and 1 times anything is the thing itself:
(define (makeproduct m1 m2)
(cond ((or (=number? m1 0) (=number? m2 0)) 0)
((=number? m1 1) m2)
((=number? m2 1) m1)
((and (number? m1) (number? m2)) (* m1 m2))
(else (list '* m1 m2))))
Here is how this version works on our three examples:
(deriv '(+ x 3) 'x)
1
(deriv '(* x y) 'x)
y
(deriv '(* (* x y) (+ x 3)) 'x)
(+ (* x y) (* y (+ x 3)))
Although this is quite an improvement, the third example shows that
there is still a long way to go before we get a program that puts
expressions into a form that we might agree is "simplest." The problem
of algebraic simplification is complex because, among other reasons, a
form that may be simplest for one purpose may not be for another.
*Exercise 2.56:* Show how to extend the basic differentiator to
handle more kinds of expressions. For instance, implement the
differentiation rule
n_1 n_2
 =  if and only if n_1 d_2 = n_2 d_1
d_1 d_2
by adding a new clause to the `deriv' program and defining
appropriate procedures `exponentiation?', `base', `exponent', and
`makeexponentiation'. (You may use the symbol `**' to denote
exponentiation.) Build in the rules that anything raised to the
power 0 is 1 and anything raised to the power 1 is the thing
itself.
*Exercise 2.57:* Extend the differentiation program to handle sums
and products of arbitrary numbers of (two or more) terms. Then
the last example above could be expressed as
(deriv '(* x y (+ x 3)) 'x)
Try to do this by changing only the representation for sums and
products, without changing the `deriv' procedure at all. For
example, the `addend' of a sum would be the first term, and the
`augend' would be the sum of the rest of the terms.
*Exercise 2.58:* Suppose we want to modify the differentiation
program so that it works with ordinary mathematical notation, in
which `+' and `*' are infix rather than prefix operators. Since
the differentiation program is defined in terms of abstract data,
we can modify it to work with different representations of
expressions solely by changing the predicates, selectors, and
constructors that define the representation of the algebraic
expressions on which the differentiator is to operate.
a. Show how to do this in order to differentiate algebraic
expressions presented in infix form, such as `(x + (3 * (x +
(y + 2))))'. To simplify the task, assume that `+' and `*'
always take two arguments and that expressions are fully
parenthesized.
b. The problem becomes substantially harder if we allow standard
algebraic notation, such as `(x + 3 * (x + y + 2))', which
drops unnecessary parentheses and assumes that multiplication
is done before addition. Can you design appropriate
predicates, selectors, and constructors for this notation
such that our derivative program still works?
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2.3.3 Example: Representing Sets

In the previous examples we built representations for two kinds of
compound data objects: rational numbers and algebraic expressions. In
one of these examples we had the choice of simplifying (reducing) the
expressions at either construction time or selection time, but other
than that the choice of a representation for these structures in terms
of lists was straightforward. When we turn to the representation of
sets, the choice of a representation is not so obvious. Indeed, there
are a number of possible representations, and they differ significantly
from one another in several ways.
Informally, a set is simply a collection of distinct objects. To
give a more precise definition we can employ the method of data
abstraction. That is, we define "set" by specifying the operations
that are to be used on sets. These are `unionset',
`intersectionset', `elementofset?', and `adjoinset'.
`Elementofset?' is a predicate that determines whether a given
element is a member of a set. `Adjoinset' takes an object and a set
as arguments and returns a set that contains the elements of the
original set and also the adjoined element. `Unionset' computes the
union of two sets, which is the set containing each element that
appears in either argument. `Intersectionset' computes the
intersection of two sets, which is the set containing only elements
that appear in both arguments. From the viewpoint of data abstraction,
we are free to design any representation that implements these
operations in a way consistent with the interpretations given above.(1)
Sets as unordered lists
.......................
One way to represent a set is as a list of its elements in which no
element appears more than once. The empty set is represented by the
empty list. In this representation, `elementofset?' is similar to
the procedure `memq' of section *Note 231::. It uses `equal?'
instead of `eq?' so that the set elements need not be symbols:
(define (elementofset? x set)
(cond ((null? set) false)
((equal? x (car set)) true)
(else (elementofset? x (cdr set)))))
Using this, we can write `adjoinset'. If the object to be adjoined
is already in the set, we just return the set. Otherwise, we use
`cons' to add the object to the list that represents the set:
(define (adjoinset x set)
(if (elementofset? x set)
set
(cons x set)))
For `intersectionset' we can use a recursive strategy. If we know
how to form the intersection of `set2' and the `cdr' of `set1', we only
need to decide whether to include the `car' of `set1' in this. But
this depends on whether `(car set1)' is also in `set2'. Here is the
resulting procedure:
(define (intersectionset set1 set2)
(cond ((or (null? set1) (null? set2)) '())
((elementofset? (car set1) set2)
(cons (car set1)
(intersectionset (cdr set1) set2)))
(else (intersectionset (cdr set1) set2))))
In designing a representation, one of the issues we should be
concerned with is efficiency. Consider the number of steps required by
our set operations. Since they all use `elementofset?', the speed of
this operation has a major impact on the efficiency of the set
implementation as a whole. Now, in order to check whether an object is
a member of a set, `elementofset?' may have to scan the entire set.
(In the worst case, the object turns out not to be in the set.) Hence,
if the set has n elements, `elementofset?' might take up to n steps.
Thus, the number of steps required grows as [theta](n). The number of
steps required by `adjoinset', which uses this operation, also grows
as [theta](n). For `intersectionset', which does an `elementofset?'
check for each element of `set1', the number of steps required grows as
the product of the sizes of the sets involved, or [theta](n^2) for two
sets of size n. The same will be true of `unionset'.
*Exercise 2.59:* Implement the `unionset' operation for the
unorderedlist representation of sets.
*Exercise 2.60:* We specified that a set would be represented as a
list with no duplicates. Now suppose we allow duplicates. For
instance, the set {1,2,3} could be represented as the list `(2 3 2
1 3 2 2)'. Design procedures `elementofset?', `adjoinset',
`unionset', and `intersectionset' that operate on this
representation. How does the efficiency of each compare with the
corresponding procedure for the nonduplicate representation? Are
there applications for which you would use this representation in
preference to the nonduplicate one?
Sets as ordered lists
.....................
One way to speed up our set operations is to change the representation
so that the set elements are listed in increasing order. To do this,
we need some way to compare two objects so that we can say which is
bigger. For example, we could compare symbols lexicographically, or we
could agree on some method for assigning a unique number to an object
and then compare the elements by comparing the corresponding numbers.
To keep our discussion simple, we will consider only the case where the
set elements are numbers, so that we can compare elements using `>' and
`<'. We will represent a set of numbers by listing its elements in
increasing order. Whereas our first representation above allowed us to
represent the set {1,3,6,10} by listing the elements in any order, our
new representation allows only the list `(1 3 6 10)'.
One advantage of ordering shows up in `elementofset?': In checking
for the presence of an item, we no longer have to scan the entire set.
If we reach a set element that is larger than the item we are looking
for, then we know that the item is not in the set:
(define (elementofset? x set)
(cond ((null? set) false)
((= x (car set)) true)
((< x (car set)) false)
(else (elementofset? x (cdr set)))))
How many steps does this save? In the worst case, the item we are
looking for may be the largest one in the set, so the number of steps
is the same as for the unordered representation. On the other hand, if
we search for items of many different sizes we can expect that
sometimes we will be able to stop searching at a point near the
beginning of the list and that other times we will still need to
examine most of the list. On the average we should expect to have to
examine about half of the items in the set. Thus, the average number
of steps required will be about n/2. This is still [theta](n) growth,
but it does save us, on the average, a factor of 2 in number of steps
over the previous implementation.
We obtain a more impressive speedup with `intersectionset'. In the
unordered representation this operation required [theta](n^2) steps,
because we performed a complete scan of `set2' for each element of
`set1'. But with the ordered representation, we can use a more clever
method. Begin by comparing the initial elements, `x1' and `x2', of the
two sets. If `x1' equals `x2', then that gives an element of the
intersection, and the rest of the intersection is the intersection of
the `cdr's of the two sets. Suppose, however, that `x1' is less than
`x2'. Since `x2' is the smallest element in `set2', we can immediately
conclude that `x1' cannot appear anywhere in `set2' and hence is not in
the intersection. Hence, the intersection is equal to the intersection
of `set2' with the `cdr' of `set1'. Similarly, if `x2' is less than
`x1', then the intersection is given by the intersection of `set1' with
the `cdr' of `set2'. Here is the procedure:
(define (intersectionset set1 set2)
(if (or (null? set1) (null? set2))
'()
(let ((x1 (car set1)) (x2 (car set2)))
(cond ((= x1 x2)
(cons x1
(intersectionset (cdr set1)
(cdr set2))))
((< x1 x2)
(intersectionset (cdr set1) set2))
((< x2 x1)
(intersectionset set1 (cdr set2)))))))
To estimate the number of steps required by this process, observe
that at each step we reduce the intersection problem to computing
intersections of smaller setsremoving the first element from `set1'
or `set2' or both. Thus, the number of steps required is at most the
sum of the sizes of `set1' and `set2', rather than the product of the
sizes as with the unordered representation. This is [theta](n) growth
rather than [theta](n^2)a considerable speedup, even for sets of
moderate size.
*Exercise 2.61:* Give an implementation of `adjoinset' using the
ordered representation. By analogy with `elementofset?' show
how to take advantage of the ordering to produce a procedure that
requires on the average about half as many steps as with the
unordered representation.
*Exercise 2.62:* Give a [theta](n) implementation of `unionset'
for sets represented as ordered lists.
Sets as binary trees
....................
We can do better than the orderedlist representation by arranging the
set elements in the form of a tree. Each node of the tree holds one
element of the set, called the "entry" at that node, and a link to each
of two other (possibly empty) nodes. The "left" link points to
elements smaller than the one at the node, and the "right" link to
elements greater than the one at the node. *Note Figure 216:: shows
some trees that represent the set {1,3,5,7,9,11}. The same set may be
represented by a tree in a number of different ways. The only thing we
require for a valid representation is that all elements in the left
subtree be smaller than the node entry and that all elements in the
right subtree be larger.
*Figure 2.16:* Various binary trees that represent the set
{1,3,5,7,9,11}.
7 3 5
/\ /\ /\
3 9 1 7 3 9
/\ \ /\ / /\
1 5 11 5 9 1 7 11
\
11
The advantage of the tree representation is this: Suppose we want to
check whether a number x is contained in a set. We begin by comparing
x with the entry in the top node. If x is less than this, we know that
we need only search the left subtree; if x is greater, we need only
search the right subtree. Now, if the tree is "balanced," each of
these subtrees will be about half the size of the original. Thus, in
one step we have reduced the problem of searching a tree of size n to
searching a tree of size n/2. Since the size of the tree is halved at
each step, we should expect that the number of steps needed to search a
tree of size n grows as [theta](`log' n).(2) For large sets, this will
be a significant speedup over the previous representations.
We can represent trees by using lists. Each node will be a list of
three items: the entry at the node, the left subtree, and the right
subtree. A left or a right subtree of the empty list will indicate
that there is no subtree connected there. We can describe this
representation by the following procedures:(3)
(define (entry tree) (car tree))
(define (leftbranch tree) (cadr tree))
(define (rightbranch tree) (caddr tree))
(define (maketree entry left right)
(list entry left right))
Now we can write the `elementofset?' procedure using the strategy
described above:
(define (elementofset? x set)
(cond ((null? set) false)
((= x (entry set)) true)
((< x (entry set))
(elementofset? x (leftbranch set)))
((> x (entry set))
(elementofset? x (rightbranch set)))))
Adjoining an item to a set is implemented similarly and also requires
[theta](`log' n) steps. To adjoin an item `x', we compare `x' with the
node entry to determine whether `x' should be added to the right or to
the left branch, and having adjoined `x' to the appropriate branch we
piece this newly constructed branch together with the original entry
and the other branch. If `x' is equal to the entry, we just return the
node. If we are asked to adjoin `x' to an empty tree, we generate a
tree that has `x' as the entry and empty right and left branches. Here
is the procedure:
(define (adjoinset x set)
(cond ((null? set) (maketree x '() '()))
((= x (entry set)) set)
((< x (entry set))
(maketree (entry set)
(adjoinset x (leftbranch set))
(rightbranch set)))
((> x (entry set))
(maketree (entry set)
(leftbranch set)
(adjoinset x (rightbranch set))))))
The above claim that searching the tree can be performed in a
logarithmic number of steps rests on the assumption that the tree is
"balanced," i.e., that the left and the right subtree of every tree
have approximately the same number of elements, so that each subtree
contains about half the elements of its parent. But how can we be
certain that the trees we construct will be balanced? Even if we start
with a balanced tree, adding elements with `adjoinset' may produce an
unbalanced result. Since the position of a newly adjoined element
depends on how the element compares with the items already in the set,
we can expect that if we add elements "randomly" the tree will tend to
be balanced on the average. But this is not a guarantee. For example,
if we start with an empty set and adjoin the numbers 1 through 7 in
sequence we end up with the highly unbalanced tree shown in *Note
Figure 217::. In this tree all the left subtrees are empty, so it has
no advantage over a simple ordered list. One way to solve this problem
is to define an operation that transforms an arbitrary tree into a
balanced tree with the same elements. Then we can perform this
transformation after every few `adjoinset' operations to keep our set
in balance. There are also other ways to solve this problem, most of
which involve designing new data structures for which searching and
insertion both can be done in [theta](`log' n) steps.(4)
*Figure 2.17:* Unbalanced tree produced by adjoining 1 through 7
in sequence.
1
\
2
\
4
\
5
\
6
\
7
*Exercise 2.63:* Each of the following two procedures converts a
binary tree to a list.
(define (tree>list1 tree)
(if (null? tree)
'()
(append (tree>list1 (leftbranch tree))
(cons (entry tree)
(tree>list1 (rightbranch tree))))))
(define (tree>list2 tree)
(define (copytolist tree resultlist)
(if (null? tree)
resultlist
(copytolist (leftbranch tree)
(cons (entry tree)
(copytolist (rightbranch tree)
resultlist)))))
(copytolist tree '()))
a. Do the two procedures produce the same result for every tree?
If not, how do the results differ? What lists do the two
procedures produce for the trees in *Note Figure 216::?
b. Do the two procedures have the same order of growth in the
number of steps required to convert a balanced tree with n
elements to a list? If not, which one grows more slowly?
*Exercise 2.64:* The following procedure `list>tree' converts an
ordered list to a balanced binary tree. The helper procedure
`partialtree' takes as arguments an integer n and list of at
least n elements and constructs a balanced tree containing the
first n elements of the list. The result returned by
`partialtree' is a pair (formed with `cons') whose `car' is the
constructed tree and whose `cdr' is the list of elements not
included in the tree.
(define (list>tree elements)
(car (partialtree elements (length elements))))
(define (partialtree elts n)
(if (= n 0)
(cons '() elts)
(let ((leftsize (quotient ( n 1) 2)))
(let ((leftresult (partialtree elts leftsize)))
(let ((lefttree (car leftresult))
(nonleftelts (cdr leftresult))
(rightsize ( n (+ leftsize 1))))
(let ((thisentry (car nonleftelts))
(rightresult (partialtree (cdr nonleftelts)
rightsize)))
(let ((righttree (car rightresult))
(remainingelts (cdr rightresult)))
(cons (maketree thisentry lefttree righttree)
remainingelts))))))))
a. Write a short paragraph explaining as clearly as you can how
`partialtree' works. Draw the tree produced by `list>tree'
for the list `(1 3 5 7 9 11)'.
b. What is the order of growth in the number of steps required by
`list>tree' to convert a list of n elements?
*Exercise 2.65:* Use the results of *Note Exercise 263:: and
*Note Exercise 264:: to give [theta](n) implementations of
`unionset' and `intersectionset' for sets implemented as
(balanced) binary trees.(5)
Sets and information retrieval
..............................
We have examined options for using lists to represent sets and have
seen how the choice of representation for a data object can have a
large impact on the performance of the programs that use the data.
Another reason for concentrating on sets is that the techniques
discussed here appear again and again in applications involving
information retrieval.
Consider a data base containing a large number of individual
records, such as the personnel files for a company or the transactions
in an accounting system. A typical datamanagement system spends a
large amount of time accessing or modifying the data in the records and
therefore requires an efficient method for accessing records. This is
done by identifying a part of each record to serve as an identifying "key".
A key can be anything that uniquely identifies the record. For a
personnel file, it might be an employee's ID number. For an accounting
system, it might be a transaction number. Whatever the key is, when we
define the record as a data structure we should include a `key'
selector procedure that retrieves the key associated with a given
record.
Now we represent the data base as a set of records. To locate the
record with a given key we use a procedure `lookup', which takes as
arguments a key and a data base and which returns the record that has
that key, or false if there is no such record. `Lookup' is implemented
in almost the same way as `elementofset?'. For example, if the set
of records is implemented as an unordered list, we could use
(define (lookup givenkey setofrecords)
(cond ((null? setofrecords) false)
((equal? givenkey (key (car setofrecords)))
(car setofrecords))
(else (lookup givenkey (cdr setofrecords)))))
Of course, there are better ways to represent large sets than as
unordered lists. Informationretrieval systems in which records have
to be "randomly accessed" are typically implemented by a treebased
method, such as the binarytree representation discussed previously.
In designing such a system the methodology of data abstraction can be a
great help. The designer can create an initial implementation using a
simple, straightforward representation such as unordered lists. This
will be unsuitable for the eventual system, but it can be useful in
providing a "quick and dirty" data base with which to test the rest of
the system. Later on, the data representation can be modified to be
more sophisticated. If the data base is accessed in terms of abstract
selectors and constructors, this change in representation will not
require any changes to the rest of the system.
*Exercise 2.66:* Implement the `lookup' procedure for the case
where the set of records is structured as a binary tree, ordered
by the numerical values of the keys.
 Footnotes 
(1) If we want to be more formal, we can specify "consistent with
the interpretations given above" to mean that the operations satisfy a
collection of rules such as these:
* For any set `S' and any object `x', `(elementofset? x
(adjoinset x S))' is true (informally: "Adjoining an object to a
set produces a set that contains the object").
* For any sets `S' and `T' and any object `x', `(elementofset? x
(unionset S T))' is equal to `(or (elementofset? x S)
(elementofset? x T))' (informally: "The elements of `(union S
T)' are the elements that are in `S' or in `T'").
* For any object `x', `(elementofset? x '())' is false
(informally: "No object is an element of the empty set").
(2) Halving the size of the problem at each step is the
distinguishing characteristic of logarithmic growth, as we saw with the
fastexponentiation algorithm of section *Note 124:: and the
halfinterval search method of section *Note 133::.
(3) We are representing sets in terms of trees, and trees in terms
of listsin effect, a data abstraction built upon a data abstraction.
We can regard the procedures `entry', `leftbranch', `rightbranch',
and `maketree' as a way of isolating the abstraction of a "binary
tree" from the particular way we might wish to represent such a tree in
terms of list structure.
(4) Examples of such structures include "Btrees" and "redblack
trees". There is a large literature on data structures devoted to this
problem. See Cormen, Leiserson, and Rivest 1990.
(5) *Note Exercise 263:: through *Note Exercise 265:: are due to
Paul Hilfinger.
File: sicp.info, Node: 234, Prev: 233, Up: 23
2.3.4 Example: Huffman Encoding Trees

This section provides practice in the use of list structure and data
abstraction to manipulate sets and trees. The application is to
methods for representing data as sequences of ones and zeros (bits).
For example, the ASCII standard code used to represent text in
computers encodes each character as a sequence of seven bits. Using
seven bits allows us to distinguish 2^(7), or 128, possible different
characters. In general, if we want to distinguish n different symbols,
we will need to use `log'_2 n bits per symbol. If all our messages are
made up of the eight symbols A, B, C, D, E, F, G, and H, we can choose
a code with three bits per character, for example
A 000 C 010 E 100 G 110
B 001 D 011 F 101 H 111
With this code, the message
BACADAEAFABBAAAGAH
is encoded as the string of 54 bits
001000010000011000100000101000001001000000000110000111
Codes such as ASCII and the AthroughH code above are known as "fixedlength"
codes, because they represent each symbol in the message with the same
number of bits. It is sometimes advantageous to use "variablelength"
codes, in which different symbols may be represented by different
numbers of bits. For example, Morse code does not use the same number
of dots and dashes for each letter of the alphabet. In particular, E,
the most frequent letter, is represented by a single dot. In general,
if our messages are such that some symbols appear very frequently and
some very rarely, we can encode data more efficiently (i.e., using
fewer bits per message) if we assign shorter codes to the frequent
symbols. Consider the following alternative code for the letters A
through H:
A 0 C 1010 E 1100 G 1110
B 100 D 1011 F 1101 H 1111
With this code, the same message as above is encoded as the string
100010100101101100011010100100000111001111
This string contains 42 bits, so it saves more than 20% in space in
comparison with the fixedlength code shown above.
One of the difficulties of using a variablelength code is knowing
when you have reached the end of a symbol in reading a sequence of
zeros and ones. Morse code solves this problem by using a special "separator
code" (in this case, a pause) after the sequence of dots and dashes for
each letter. Another solution is to design the code in such a way that
no complete code for any symbol is the beginning (or "prefix") of the
code for another symbol. Such a code is called a "prefix code". In
the example above, A is encoded by 0 and B is encoded by 100, so no
other symbol can have a code that begins with 0 or with 100.
In general, we can attain significant savings if we use
variablelength prefix codes that take advantage of the relative
frequencies of the symbols in the messages to be encoded. One
particular scheme for doing this is called the Huffman encoding method,
after its discoverer, David Huffman. A Huffman code can be represented
as a binary tree whose leaves are the symbols that are encoded. At
each nonleaf node of the tree there is a set containing all the
symbols in the leaves that lie below the node. In addition, each
symbol at a leaf is assigned a weight (which is its relative
frequency), and each nonleaf node contains a weight that is the sum of
all the weights of the leaves lying below it. The weights are not used
in the encoding or the decoding process. We will see below how they
are used to help construct the tree.
*Figure 2.18:* A Huffman encoding tree.
{A B C D E F G H} 17
*
/ \
/ \
A 8 * {B C D E F G H} 9
__________/ \_____________
/ \
{B C D} 5 * * {E F G H} 4
/ \ ___/ \___
/ \ / \
B 3 * {C D} 2 {E F} 2 * * {G H} 2
/ \ / \ / \
/ \ / \ / \
C 1 D 1 E 1 F 1 G 1 H 1
*Note Figure 218:: shows the Huffman tree for the AthroughH code
given above. The weights at the leaves indicate that the tree was
designed for messages in which A appears with relative frequency 8, B
with relative frequency 3, and the other letters each with relative
frequency 1.
Given a Huffman tree, we can find the encoding of any symbol by
starting at the root and moving down until we reach the leaf that holds
the symbol. Each time we move down a left branch we add a 0 to the
code, and each time we move down a right branch we add a 1. (We decide
which branch to follow by testing to see which branch either is the
leaf node for the symbol or contains the symbol in its set.) For
example, starting from the root of the tree in *Note Figure 218::, we
arrive at the leaf for D by following a right branch, then a left
branch, then a right branch, then a right branch; hence, the code for D
is 1011.
To decode a bit sequence using a Huffman tree, we begin at the root
and use the successive zeros and ones of the bit sequence to determine
whether to move down the left or the right branch. Each time we come
to a leaf, we have generated a new symbol in the message, at which
point we start over from the root of the tree to find the next symbol.
For example, suppose we are given the tree above and the sequence
10001010. Starting at the root, we move down the right branch, (since
the first bit of the string is 1), then down the left branch (since the
second bit is 0), then down the left branch (since the third bit is
also 0). This brings us to the leaf for B, so the first symbol of the
decoded message is B. Now we start again at the root, and we make a
left move because the next bit in the string is 0. This brings us to
the leaf for A. Then we start again at the root with the rest of the
string 1010, so we move right, left, right, left and reach C. Thus,
the entire message is BAC.
Generating Huffman trees
........................
Given an "alphabet" of symbols and their relative frequencies, how do we
construct the "best" code? (In other words, which tree will encode
messages with the fewest bits?) Huffman gave an algorithm for doing
this and showed that the resulting code is indeed the best
variablelength code for messages where the relative frequency of the
symbols matches the frequencies with which the code was constructed.
We will not prove this optimality of Huffman codes here, but we will
show how Huffman trees are constructed.(1)
The algorithm for generating a Huffman tree is very simple. The idea
is to arrange the tree so that the symbols with the lowest frequency
appear farthest away from the root. Begin with the set of leaf nodes,
containing symbols and their frequencies, as determined by the initial
data from which the code is to be constructed. Now find two leaves with
the lowest weights and merge them to produce a node that has these two
nodes as its left and right branches. The weight of the new node is the
sum of the two weights. Remove the two leaves from the original set and
replace them by this new node. Now continue this process. At each step,
merge two nodes with the smallest weights, removing them from the set
and replacing them with a node that has these two as its left and right
branches. The process stops when there is only one node left, which is
the root of the entire tree. Here is how the Huffman tree of *Note
Figure 218:: was generated:
Initial leaves {(A 8) (B 3) (C 1) (D 1) (E 1) (F 1) (G 1) (H 1)}
Merge {(A 8) (B 3) ({C D} 2) (E 1) (F 1) (G 1) (H 1)}
Merge {(A 8) (B 3) ({C D} 2) ({E F} 2) (G 1) (H 1)}
Merge {(A 8) (B 3) ({C D} 2) ({E F} 2) ({G H} 2)}
Merge {(A 8) (B 3) ({C D} 2) ({E F G H} 4)}
Merge {(A 8) ({B C D} 5) ({E F G H} 4)}
Merge {(A 8) ({B C D E F G H} 9)}
Final merge {({A B C D E F G H} 17)}
The algorithm does not always specify a unique tree, because there
may not be unique smallestweight nodes at each step. Also, the choice
of the order in which the two nodes are merged (i.e., which will be the
right branch and which will be the left branch) is arbitrary.
Representing Huffman trees
..........................
In the exercises below we will work with a system that uses Huffman
trees to encode and decode messages and generates Huffman trees
according to the algorithm outlined above. We will begin by discussing
how trees are represented.
Leaves of the tree are represented by a list consisting of the symbol
`leaf', the symbol at the leaf, and the weight:
(define (makeleaf symbol weight)
(list 'leaf symbol weight))
(define (leaf? object)
(eq? (car object) 'leaf))
(define (symbolleaf x) (cadr x))
(define (weightleaf x) (caddr x))
A general tree will be a list of a left branch, a right branch, a
set of symbols, and a weight. The set of symbols will be simply a list
of the symbols, rather than some more sophisticated set representation.
When we make a tree by merging two nodes, we obtain the weight of the
tree as the sum of the weights of the nodes, and the set of symbols as
the union of the sets of symbols for the nodes. Since our symbol sets
are represented as lists, we can form the union by using the `append'
procedure we defined in section *Note 221:::
(define (makecodetree left right)
(list left
right
(append (symbols left) (symbols right))
(+ (weight left) (weight right))))
If we make a tree in this way, we have the following selectors:
(define (leftbranch tree) (car tree))
(define (rightbranch tree) (cadr tree))
(define (symbols tree)
(if (leaf? tree)
(list (symbolleaf tree))
(caddr tree)))
(define (weight tree)
(if (leaf? tree)
(weightleaf tree)
(cadddr tree)))
The procedures `symbols' and `weight' must do something slightly
different depending on whether they are called with a leaf or a general
tree. These are simple examples of "generic procedures" (procedures
that can handle more than one kind of data), which we will have much
more to say about in sections *Note 24:: and *Note 25::.
The decoding procedure
......................
The following procedure implements the decoding algorithm. It takes as
arguments a list of zeros and ones, together with a Huffman tree.
(define (decode bits tree)
(define (decode1 bits currentbranch)
(if (null? bits)
'()
(let ((nextbranch
(choosebranch (car bits) currentbranch)))
(if (leaf? nextbranch)
(cons (symbolleaf nextbranch)
(decode1 (cdr bits) tree))
(decode1 (cdr bits) nextbranch)))))
(decode1 bits tree))
(define (choosebranch bit branch)
(cond ((= bit 0) (leftbranch branch))
((= bit 1) (rightbranch branch))
(else (error "bad bit  CHOOSEBRANCH" bit))))
The procedure `decode1' takes two arguments: the list of remaining
bits and the current position in the tree. It keeps moving "down" the
tree, choosing a left or a right branch according to whether the next
bit in the list is a zero or a one. (This is done with the procedure
`choosebranch'.) When it reaches a leaf, it returns the symbol at
that leaf as the next symbol in the message by `cons'ing it onto the
result of decoding the rest of the message, starting at the root of the
tree. Note the error check in the final clause of `choosebranch',
which complains if the procedure finds something other than a zero or a
one in the input data.
Sets of weighted elements
.........................
In our representation of trees, each nonleaf node contains a set of
symbols, which we have represented as a simple list. However, the
treegenerating algorithm discussed above requires that we also work
with sets of leaves and trees, successively merging the two smallest
items. Since we will be required to repeatedly find the smallest item
in a set, it is convenient to use an ordered representation for this
kind of set.
We will represent a set of leaves and trees as a list of elements,
arranged in increasing order of weight. The following `adjoinset'
procedure for constructing sets is similar to the one described in
*Note Exercise 261::; however, items are compared by their weights,
and the element being added to the set is never already in it.
(define (adjoinset x set)
(cond ((null? set) (list x))
((< (weight x) (weight (car set))) (cons x set))
(else (cons (car set)
(adjoinset x (cdr set))))))
The following procedure takes a list of symbolfrequency pairs such
as `((A 4) (B 2) (C 1) (D 1))' and constructs an initial ordered set of
leaves, ready to be merged according to the Huffman algorithm:
(define (makeleafset pairs)
(if (null? pairs)
'()
(let ((pair (car pairs)))
(adjoinset (makeleaf (car pair) ; symbol
(cadr pair)) ; frequency
(makeleafset (cdr pairs))))))
*Exercise 2.67:* Define an encoding tree and a sample message:
(define sampletree
(makecodetree (makeleaf 'A 4)
(makecodetree
(makeleaf 'B 2)
(makecodetree (makeleaf 'D 1)
(makeleaf 'C 1)))))
(define samplemessage '(0 1 1 0 0 1 0 1 0 1 1 1 0))
Use the `decode' procedure to decode the message, and give the
result.
*Exercise 2.68:* The `encode' procedure takes as arguments a
message and a tree and produces the list of bits that gives the
encoded message.
(define (encode message tree)
(if (null? message)
'()
(append (encodesymbol (car message) tree)
(encode (cdr message) tree))))
`Encodesymbol' is a procedure, which you must write, that returns
the list of bits that encodes a given symbol according to a given
tree. You should design `encodesymbol' so that it signals an
error if the symbol is not in the tree at all. Test your
procedure by encoding the result you obtained in *Note Exercise
267:: with the sample tree and seeing whether it is the same as
the original sample message.
*Exercise 2.69:* The following procedure takes as its argument a
list of symbolfrequency pairs (where no symbol appears in more
than one pair) and generates a Huffman encoding tree according to
the Huffman algorithm.
(define (generatehuffmantree pairs)
(successivemerge (makeleafset pairs)))
`Makeleafset' is the procedure given above that transforms the
list of pairs into an ordered set of leaves. `Successivemerge'
is the procedure you must write, using `makecodetree' to
successively merge the smallestweight elements of the set until
there is only one element left, which is the desired Huffman tree.
(This procedure is slightly tricky, but not really complicated.
If you find yourself designing a complex procedure, then you are
almost certainly doing something wrong. You can take significant
advantage of the fact that we are using an ordered set
representation.)
*Exercise 2.70:* The following eightsymbol alphabet with
associated relative frequencies was designed to efficiently encode
the lyrics of 1950s rock songs. (Note that the "symbols" of an
"alphabet" need not be individual letters.)
A 2 NA 16
BOOM 1 SHA 3
GET 2 YIP 9
JOB 2 WAH 1
Use `generatehuffmantree' (*Note Exercise 269::) to generate a
corresponding Huffman tree, and use `encode' (*Note Exercise
268::) to encode the following message:
Get a job
Sha na na na na na na na na
Get a job
Sha na na na na na na na na
Wah yip yip yip yip yip yip yip yip yip
Sha boom
How many bits are required for the encoding? What is the smallest
number of bits that would be needed to encode this song if we used
a fixedlength code for the eightsymbol alphabet?
*Exercise 2.71:* Suppose we have a Huffman tree for an alphabet of
n symbols, and that the relative frequencies of the symbols are 1,
2, 4, ..., 2^(n1). Sketch the tree for n=5; for n=10. In such a
tree (for general n) how may bits are required to encode the most
frequent symbol? the least frequent symbol?
*Exercise 2.72:* Consider the encoding procedure that you designed
in *Note Exercise 268::. What is the order of growth in the
number of steps needed to encode a symbol? Be sure to include the
number of steps needed to search the symbol list at each node
encountered. To answer this question in general is difficult.
Consider the special case where the relative frequencies of the n
symbols are as described in *Note Exercise 271::, and give the
order of growth (as a function of n) of the number of steps needed
to encode the most frequent and least frequent symbols in the
alphabet.
 Footnotes 
(1) See Hamming 1980 for a discussion of the mathematical properties
of Huffman codes.
File: sicp.info, Node: 24, Next: 25, Prev: 23, Up: Chapter 2
2.4 Multiple Representations for Abstract Data
==============================================
We have introduced data abstraction, a methodology for structuring
systems in such a way that much of a program can be specified
independent of the choices involved in implementing the data objects
that the program manipulates. For example, we saw in section *Note
211:: how to separate the task of designing a program that uses
rational numbers from the task of implementing rational numbers in
terms of the computer language's primitive mechanisms for constructing
compound data. The key idea was to erect an abstraction barrier  in
this case, the selectors and constructors for rational numbers
(`makerat', `numer', `denom')that isolates the way rational numbers
are used from their underlying representation in terms of list
structure. A similar abstraction barrier isolates the details of the
procedures that perform rational arithmetic (`addrat', `subrat',
`mulrat', and `divrat') from the "higherlevel" procedures that use
rational numbers. The resulting program has the structure shown in
*Note Figure 21::.
These dataabstraction barriers are powerful tools for controlling
complexity. By isolating the underlying representations of data
objects, we can divide the task of designing a large program into
smaller tasks that can be performed separately. But this kind of data
abstraction is not yet powerful enough, because it may not always make
sense to speak of "the underlying representation" for a data object.
For one thing, there might be more than one useful representation
for a data object, and we might like to design systems that can deal
with multiple representations. To take a simple example, complex
numbers may be represented in two almost equivalent ways: in
rectangular form (real and imaginary parts) and in polar form
(magnitude and angle). Sometimes rectangular form is more appropriate
and sometimes polar form is more appropriate. Indeed, it is perfectly
plausible to imagine a system in which complex numbers are represented
in both ways, and in which the procedures for manipulating complex
numbers work with either representation.
More importantly, programming systems are often designed by many
people working over extended periods of time, subject to requirements
that change over time. In such an environment, it is simply not
possible for everyone to agree in advance on choices of data
representation. So in addition to the dataabstraction barriers that
isolate representation from use, we need abstraction barriers that
isolate different design choices from each other and permit different
choices to coexist in a single program. Furthermore, since large
programs are often created by combining preexisting modules that were
designed in isolation, we need conventions that permit programmers to
incorporate modules into larger systems "additively", that is, without
having to redesign or reimplement these modules.
In this section, we will learn how to cope with data that may be
represented in different ways by different parts of a program. This
requires constructing "generic procedures"procedures that can operate
on data that may be represented in more than one way. Our main
technique for building generic procedures will be to work in terms of
data objects that have tags "type tags", that is, data objects that
include explicit information about how they are to be processed. We
will also discuss "datadirected" programming, a powerful and
convenient implementation strategy for additively assembling systems
with generic operations.
We begin with the simple complexnumber example. We will see how
type tags and datadirected style enable us to design separate
rectangular and polar representations for complex numbers while
maintaining the notion of an abstract "complexnumber" data object. We
will accomplish this by defining arithmetic procedures for complex
numbers (`addcomplex', `subcomplex', `mulcomplex', and
`divcomplex') in terms of generic selectors that access parts of a
complex number independent of how the number is represented. The
resulting complexnumber system, as shown in *Note Figure 219::,
contains two different kinds of abstraction barriers. The "horizontal"
abstraction barriers play the same role as the ones in *Note Figure
21::. They isolate "higherlevel" operations from "lowerlevel"
representations. In addition, there is a "vertical" barrier that gives
us the ability to separately design and install alternative
representations.
*Figure 2.19:* Dataabstraction barriers in the complexnumber
system.
Programs that use complex numbers
++
 addcomplex subcomplex mulcomplex divcomplex 
++
Complex arithmetic package
+
Rectangular  Polar
representation  representation
+
List structure and primitive machine arithmetic
In section *Note 25:: we will show how to use type tags and
datadirected style to develop a generic arithmetic package. This
provides procedures (`add', `mul', and so on) that can be used to
manipulate all sorts of "numbers" and can be easily extended when a new
kind of number is needed. In section *Note 253::, we'll show how to
use generic arithmetic in a system that performs symbolic algebra.
* Menu:
* 241:: Representations for Complex Numbers
* 242:: Tagged data
* 243:: DataDirected Programming and Additivity
File: sicp.info, Node: 241, Next: 242, Prev: 24, Up: 24
2.4.1 Representations for Complex Numbers

We will develop a system that performs arithmetic operations on complex
numbers as a simple but unrealistic example of a program that uses
generic operations. We begin by discussing two plausible
representations for complex numbers as ordered pairs: rectangular form
(real part and imaginary part) and polar form (magnitude and angle).(1)
Section *Note 242:: will show how both representations can be made
to coexist in a single system through the use of type tags and generic
operations.
Like rational numbers, complex numbers are naturally represented as
ordered pairs. The set of complex numbers can be thought of as a
twodimensional space with two orthogonal axes, the "real" axis and the
"imaginary" axis. (See *Note Figure 220::.) From this point of view,
the complex number z = x + iy (where i^2 =  1) can be thought of as
the point in the plane whose real coordinate is x and whose imaginary
coordinate is y. Addition of complex numbers reduces in this
representation to addition of coordinates:
Realpart(z_1 + z_2) = Realpart(z_1) + Realpart(z_2)
Imaginarypart(z_1 + z_2) = Imaginarypart(z_1) + Imaginarypart(z_2)
When multiplying complex numbers, it is more natural to think in
terms of representing a complex number in polar form, as a magnitude
and an angle (r and A in *Note Figure 220::). The product of two
complex numbers is the vector obtained by stretching one complex number
by the length of the other and then rotating it through the angle of
the other:
Magnitude(z_1 * z_2) = Magnitude(z_1) * Magnitude(z_2)
Angle(z_1 * z_2) = Angle(z_1) + Angle(z_2)
*Figure 2.20:* Complex numbers as points in the plane.
Imaginary
^

y .........................* z = x + ?y = r e^(?A)
 __ .
 __ .
 r __ .
 __ .
 __ \ .
__ A  .
++> Real
x
Thus, there are two different representations for complex numbers,
which are appropriate for different operations. Yet, from the
viewpoint of someone writing a program that uses complex numbers, the
principle of data abstraction suggests that all the operations for
manipulating complex numbers should be available regardless of which
representation is used by the computer. For example, it is often
useful to be able to find the magnitude of a complex number that is
specified by rectangular coordinates. Similarly, it is often useful to
be able to determine the real part of a complex number that is
specified by polar coordinates.
To design such a system, we can follow the same dataabstraction
strategy we followed in designing the rationalnumber package in
section *Note 211::. Assume that the operations on complex numbers
are implemented in terms of four selectors: `realpart', `imagpart',
`magnitude', and `angle'. Also assume that we have two procedures for
constructing complex numbers: `makefromrealimag' returns a complex
number with specified real and imaginary parts, and `makefrommagang'
returns a complex number with specified magnitude and angle. These
procedures have the property that, for any complex number `z', both
(makefromrealimag (realpart z) (imagpart z))
and
(makefrommagang (magnitude z) (angle z))
produce complex numbers that are equal to `z'.
Using these constructors and selectors, we can implement arithmetic
on complex numbers using the "abstract data" specified by the
constructors and selectors, just as we did for rational numbers in
section *Note 211::. As shown in the formulas above, we can add and
subtract complex numbers in terms of real and imaginary parts while
multiplying and dividing complex numbers in terms of magnitudes and
angles:
(define (addcomplex z1 z2)
(makefromrealimag (+ (realpart z1) (realpart z2))
(+ (imagpart z1) (imagpart z2))))
(define (subcomplex z1 z2)
(makefromrealimag ( (realpart z1) (realpart z2))
( (imagpart z1) (imagpart z2))))
(define (mulcomplex z1 z2)
(makefrommagang (* (magnitude z1) (magnitude z2))
(+ (angle z1) (angle z2))))
(define (divcomplex z1 z2)
(makefrommagang (/ (magnitude z1) (magnitude z2))
( (angle z1) (angle z2))))
To complete the complexnumber package, we must choose a
representation and we must implement the constructors and selectors in
terms of primitive numbers and primitive list structure. There are two
obvious ways to do this: We can represent a complex number in
"rectangular form" as a pair (real part, imaginary part) or in "polar
form" as a pair (magnitude, angle). Which shall we choose?
In order to make the different choices concrete, imagine that there
are two programmers, Ben Bitdiddle and Alyssa P. Hacker, who are
independently designing representations for the complexnumber system.
Ben chooses to represent complex numbers in rectangular form. With
this choice, selecting the real and imaginary parts of a complex number
is straightforward, as is constructing a complex number with given real
and imaginary parts. To find the magnitude and the angle, or to
construct a complex number with a given magnitude and angle, he uses
the trigonometric relations
__________
x = r cos A r = ./ x^2 + y^2
y = r sin A A = arctan(y,x)
which relate the real and imaginary parts (x, y) to the magnitude and
the angle (r, A).(2) Ben's representation is therefore given by the
following selectors and constructors:
(define (realpart z) (car z))
(define (imagpart z) (cdr z))
(define (magnitude z)
(sqrt (+ (square (realpart z)) (square (imagpart z)))))
(define (angle z)
(atan (imagpart z) (realpart z)))
(define (makefromrealimag x y) (cons x y))
(define (makefrommagang r a)
(cons (* r (cos a)) (* r (sin a))))
Alyssa, in contrast, chooses to represent complex numbers in polar
form. For her, selecting the magnitude and angle is straightforward,
but she has to use the trigonometric relations to obtain the real and
imaginary parts. Alyssa's representation is:
(define (realpart z)
(* (magnitude z) (cos (angle z))))
(define (imagpart z)
(* (magnitude z) (sin (angle z))))
(define (magnitude z) (car z))
(define (angle z) (cdr z))
(define (makefromrealimag x y)
(cons (sqrt (+ (square x) (square y)))
(atan y x)))
(define (makefrommagang r a) (cons r a))
The discipline of data abstraction ensures that the same
implementation of `addcomplex', `subcomplex', `mulcomplex', and
`divcomplex' will work with either Ben's representation or Alyssa's
representation.
 Footnotes 
(1) In actual computational systems, rectangular form is preferable
to polar form most of the time because of roundoff errors in conversion
between rectangular and polar form. This is why the complexnumber
example is unrealistic. Nevertheless, it provides a clear illustration
of the design of a system using generic operations and a good
introduction to the more substantial systems to be developed later in
this chapter.
(2) The arctangent function referred to here, computed by Scheme's
`atan' procedure, is defined so as to take two arguments y and x and to
return the angle whose tangent is y/x. The signs of the arguments
determine the quadrant of the angle.
File: sicp.info, Node: 242, Next: 243, Prev: 241, Up: 24
2.4.2 Tagged data

One way to view data abstraction is as an application of the "principle
of least commitment." In implementing the complexnumber system in
section *Note 241::, we can use either Ben's rectangular
representation or Alyssa's polar representation. The abstraction
barrier formed by the selectors and constructors permits us to defer to
the last possible moment the choice of a concrete representation for
our data objects and thus retain maximum flexibility in our system
design.
The principle of least commitment can be carried to even further
extremes. If we desire, we can maintain the ambiguity of
representation even _after_ we have designed the selectors and
constructors, and elect to use both Ben's representation _and_ Alyssa's
representation. If both representations are included in a single
system, however, we will need some way to distinguish data in polar
form from data in rectangular form. Otherwise, if we were asked, for
instance, to find the `magnitude' of the pair (3,4), we wouldn't know
whether to answer 5 (interpreting the number in rectangular form) or 3
(interpreting the number in polar form). A straightforward way to
accomplish this distinction is to include a "type tag"the symbol
`rectangular' or `polar'as part of each complex number. Then when we
need to manipulate a complex number we can use the tag to decide which
selector to apply.
In order to manipulate tagged data, we will assume that we have
procedures `typetag' and `contents' that extract from a data object
the tag and the actual contents (the polar or rectangular coordinates,
in the case of a complex number). We will also postulate a procedure
`attachtag' that takes a tag and contents and produces a tagged data
object. A straightforward way to implement this is to use ordinary
list structure:
(define (attachtag typetag contents)
(cons typetag contents))
(define (typetag datum)
(if (pair? datum)
(car datum)
(error "Bad tagged datum  TYPETAG" datum)))
(define (contents datum)
(if (pair? datum)
(cdr datum)
(error "Bad tagged datum  CONTENTS" datum)))
Using these procedures, we can define predicates `rectangular?' and
`polar?', which recognize polar and rectangular numbers, respectively:
(define (rectangular? z)
(eq? (typetag z) 'rectangular))
(define (polar? z)
(eq? (typetag z) 'polar))
With type tags, Ben and Alyssa can now modify their code so that
their two different representations can coexist in the same system.
Whenever Ben constructs a complex number, he tags it as rectangular.
Whenever Alyssa constructs a complex number, she tags it as polar. In
addition, Ben and Alyssa must make sure that the names of their
procedures do not conflict. One way to do this is for Ben to append
the suffix `rectangular' to the name of each of his representation
procedures and for Alyssa to append `polar' to the names of hers. Here
is Ben's revised rectangular representation from section *Note 241:::
(define (realpartrectangular z) (car z))
(define (imagpartrectangular z) (cdr z))
(define (magnituderectangular z)
(sqrt (+ (square (realpartrectangular z))
(square (imagpartrectangular z)))))
(define (anglerectangular z)
(atan (imagpartrectangular z)
(realpartrectangular z)))
(define (makefromrealimagrectangular x y)
(attachtag 'rectangular (cons x y)))
(define (makefrommagangrectangular r a)
(attachtag 'rectangular
(cons (* r (cos a)) (* r (sin a)))))
and here is Alyssa's revised polar representation:
(define (realpartpolar z)
(* (magnitudepolar z) (cos (anglepolar z))))
(define (imagpartpolar z)
(* (magnitudepolar z) (sin (anglepolar z))))
(define (magnitudepolar z) (car z))
(define (anglepolar z) (cdr z))
(define (makefromrealimagpolar x y)
(attachtag 'polar
(cons (sqrt (+ (square x) (square y)))
(atan y x))))
(define (makefrommagangpolar r a)
(attachtag 'polar (cons r a)))
Each generic selector is implemented as a procedure that checks the
tag of its argument and calls the appropriate procedure for handling
data of that type. For example, to obtain the real part of a complex
number, `realpart' examines the tag to determine whether to use Ben's
`realpartrectangular' or Alyssa's `realpartpolar'. In either case,
we use `contents' to extract the bare, untagged datum and send this to
the rectangular or polar procedure as required:
(define (realpart z)
(cond ((rectangular? z)
(realpartrectangular (contents z)))
((polar? z)
(realpartpolar (contents z)))
(else (error "Unknown type  REALPART" z))))
(define (imagpart z)
(cond ((rectangular? z)
(imagpartrectangular (contents z)))
((polar? z)
(imagpartpolar (contents z)))
(else (error "Unknown type  IMAGPART" z))))
(define (magnitude z)
(cond ((rectangular? z)
(magnituderectangular (contents z)))
((polar? z)
(magnitudepolar (contents z)))
(else (error "Unknown type  MAGNITUDE" z))))
(define (angle z)
(cond ((rectangular? z)
(anglerectangular (contents z)))
((polar? z)
(anglepolar (contents z)))
(else (error "Unknown type  ANGLE" z))))
To implement the complexnumber arithmetic operations, we can use
the same procedures `addcomplex', `subcomplex', `mulcomplex', and
`divcomplex' from section *Note 241::, because the selectors they
call are generic, and so will work with either representation. For
example, the procedure `addcomplex' is still
(define (addcomplex z1 z2)
(makefromrealimag (+ (realpart z1) (realpart z2))
(+ (imagpart z1) (imagpart z2))))
Finally, we must choose whether to construct complex numbers using
Ben's representation or Alyssa's representation. One reasonable choice
is to construct rectangular numbers whenever we have real and imaginary
parts and to construct polar numbers whenever we have magnitudes and
angles:
(define (makefromrealimag x y)
(makefromrealimagrectangular x y))
(define (makefrommagang r a)
(makefrommagangpolar r a))
*Figure 2.21:* Structure of the generic complexarithmetic system.
++
 addcomplex subcomplex mulcomplex divcomplex 
++
Complex arithmetic package
++
 realpart imagpart 
 
 magnitude angle 
+++
Rectangular  Polar
representation  representation
+
List structure and primitive machine arithmetic
The resulting complexnumber system has the structure shown in *Note
Figure 221::. The system has been decomposed into three relatively
independent parts: the complexnumberarithmetic operations, Alyssa's
polar implementation, and Ben's rectangular implementation. The polar
and rectangular implementations could have been written by Ben and
Alyssa working separately, and both of these can be used as underlying
representations by a third programmer implementing the
complexarithmetic procedures in terms of the abstract
constructor/selector interface.
Since each data object is tagged with its type, the selectors
operate on the data in a generic manner. That is, each selector is
defined to have a behavior that depends upon the particular type of
data it is applied to. Notice the general mechanism for interfacing
the separate representations: Within a given representation
implementation (say, Alyssa's polar package) a complex number is an
untyped pair (magnitude, angle). When a generic selector operates on a
number of `polar' type, it strips off the tag and passes the contents on
to Alyssa's code. Conversely, when Alyssa constructs a number for
general use, she tags it with a type so that it can be appropriately
recognized by the higherlevel procedures. This discipline of
stripping off and attaching tags as data objects are passed from level
to level can be an important organizational strategy, as we shall see
in section *Note 25::.
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2.4.3 DataDirected Programming and Additivity

The general strategy of checking the type of a datum and calling an
appropriate procedure is called "dispatching on type". This is a
powerful strategy for obtaining modularity in system design. Oh the
other hand, implementing the dispatch as in section *Note 242:: has
two significant weaknesses. One weakness is that the generic interface
procedures (`realpart', `imagpart', `magnitude', and `angle') must
know about all the different representations. For instance, suppose we
wanted to incorporate a new representation for complex numbers into our
complexnumber system. We would need to identify this new
representation with a type, and then add a clause to each of the
generic interface procedures to check for the new type and apply the
appropriate selector for that representation.
Another weakness of the technique is that even though the individual
representations can be designed separately, we must guarantee that no
two procedures in the entire system have the same name. This is why
Ben and Alyssa had to change the names of their original procedures
from section *Note 241::.
The issue underlying both of these weaknesses is that the technique
for implementing generic interfaces is not "additive". The person
implementing the generic selector procedures must modify those
procedures each time a new representation is installed, and the people
interfacing the individual representations must modify their code to
avoid name conflicts. In each of these cases, the changes that must be
made to the code are straightforward, but they must be made
nonetheless, and this is a source of inconvenience and error. This is
not much of a problem for the complexnumber system as it stands, but
suppose there were not two but hundreds of different representations
for complex numbers. And suppose that there were many generic
selectors to be maintained in the abstractdata interface. Suppose, in
fact, that no one programmer knew all the interface procedures or all
the representations. The problem is real and must be addressed in such
programs as largescale databasemanagement systems.
What we need is a means for modularizing the system design even
further. This is provided by the programming technique known as programming
"datadirected programming". To understand how datadirected
programming works, begin with the observation that whenever we deal
with a set of generic operations that are common to a set of different
types we are, in effect, dealing with a twodimensional table that
contains the possible operations on one axis and the possible types on
the other axis. The entries in the table are the procedures that
implement each operation for each type of argument presented. In the
complexnumber system developed in the previous section, the
correspondence between operation name, data type, and actual procedure
was spread out among the various conditional clauses in the generic
interface procedures. But the same information could have been
organized in a table, as shown in *Note Figure 222::.
Datadirected programming is the technique of designing programs to
work with such a table directly. Previously, we implemented the
mechanism that interfaces the complexarithmetic code with the two
representation packages as a set of procedures that each perform an
explicit dispatch on type. Here we will implement the interface as a
single procedure that looks up the combination of the operation name
and argument type in the table to find the correct procedure to apply,
and then applies it to the contents of the argument. If we do this,
then to add a new representation package to the system we need not
change any existing procedures; we need only add new entries to the
table.
*Figure 2.22:* Table of operations for the complexnumber system.
 Types
Operations  Polar  Rectangular
===========+=================+======================
realpart  realpartpolar  realpartrectangular
imagpart  imagpartpolar  imagpartrectangular
magnitude  magnitudepolar  magnituderectangular
angle  anglepolar  anglerectangular
To implement this plan, assume that we have two procedures, `put' and
`get', for manipulating the operationandtype table:
* `(put  )' installs the `
 ' in the table,
indexed by the `' and the `'.
* `(get )' looks up the `', `' entry in the
table and returns the item found there. If no item is found,
`get' returns false.
For now, we can assume that `put' and `get' are included in our
language. In *Note Chapter 3:: (section *Note 333::, *Note Exercise
324::) we will see how to implement these and other operations for
manipulating tables.
Here is how datadirected programming can be used in the
complexnumber system. Ben, who developed the rectangular
representation, implements his code just as he did originally. He
defines a collection of procedures, or a "package", and interfaces
these to the rest of the system by adding entries to the table that
tell the system how to operate on rectangular numbers. This is
accomplished by calling the following procedure:
(define (installrectangularpackage)
;; internal procedures
(define (realpart z) (car z))
(define (imagpart z) (cdr z))
(define (makefromrealimag x y) (cons x y))
(define (magnitude z)
(sqrt (+ (square (realpart z))
(square (imagpart z)))))
(define (angle z)
(atan (imagpart z) (realpart z)))
(define (makefrommagang r a)
(cons (* r (cos a)) (* r (sin a))))
;; interface to the rest of the system
(define (tag x) (attachtag 'rectangular x))
(put 'realpart '(rectangular) realpart)
(put 'imagpart '(rectangular) imagpart)
(put 'magnitude '(rectangular) magnitude)
(put 'angle '(rectangular) angle)
(put 'makefromrealimag 'rectangular
(lambda (x y) (tag (makefromrealimag x y))))
(put 'makefrommagang 'rectangular
(lambda (r a) (tag (makefrommagang r a))))
'done)
Notice that the internal procedures here are the same procedures
from section *Note 241:: that Ben wrote when he was working in
isolation. No changes are necessary in order to interface them to the
rest of the system. Moreover, since these procedure definitions are
internal to the installation procedure, Ben needn't worry about name
conflicts with other procedures outside the rectangular package. To
interface these to the rest of the system, Ben installs his `realpart'
procedure under the operation name `realpart' and the type
`(rectangular)', and similarly for the other selectors.(1) The
interface also defines the constructors to be used by the external
system.(2) These are identical to Ben's internally defined
constructors, except that they attach the tag.
Alyssa's polar package is analogous:
(define (installpolarpackage)
;; internal procedures
(define (magnitude z) (car z))
(define (angle z) (cdr z))
(define (makefrommagang r a) (cons r a))
(define (realpart z)
(* (magnitude z) (cos (angle z))))
(define (imagpart z)
(* (magnitude z) (sin (angle z))))
(define (makefromrealimag x y)
(cons (sqrt (+ (square x) (square y)))
(atan y x)))
;; interface to the rest of the system
(define (tag x) (attachtag 'polar x))
(put 'realpart '(polar) realpart)
(put 'imagpart '(polar) imagpart)
(put 'magnitude '(polar) magnitude)
(put 'angle '(polar) angle)
(put 'makefromrealimag 'polar
(lambda (x y) (tag (makefromrealimag x y))))
(put 'makefrommagang 'polar
(lambda (r a) (tag (makefrommagang r a))))
'done)
Even though Ben and Alyssa both still use their original procedures
defined with the same names as each other's (e.g., `realpart'), these
definitions are now internal to different procedures (see section *Note
118::), so there is no name conflict.
The complexarithmetic selectors access the table by means of a
general "operation" procedure called `applygeneric', which applies a
generic operation to some arguments. `Applygeneric' looks in the
table under the name of the operation and the types of the arguments
and applies the resulting procedure if one is present:(3)
(define (applygeneric op . args)
(let ((typetags (map typetag args)))
(let ((proc (get op typetags)))
(if proc
(apply proc (map contents args))
(error
"No method for these types  APPLYGENERIC"
(list op typetags))))))
Using `applygeneric', we can define our generic selectors as
follows:
(define (realpart z) (applygeneric 'realpart z))
(define (imagpart z) (applygeneric 'imagpart z))
(define (magnitude z) (applygeneric 'magnitude z))
(define (angle z) (applygeneric 'angle z))
Observe that these do not change at all if a new representation is
added to the system.
We can also extract from the table the constructors to be used by
the programs external to the packages in making complex numbers from
real and imaginary parts and from magnitudes and angles. As in section
*Note 242::, we construct rectangular numbers whenever we have real
and imaginary parts, and polar numbers whenever we have magnitudes and
angles:
(define (makefromrealimag x y)
((get 'makefromrealimag 'rectangular) x y))
(define (makefrommagang r a)
((get 'makefrommagang 'polar) r a))
*Exercise 2.73:* Section *Note 232:: described a program that
performs symbolic differentiation:
(define (deriv exp var)
(cond ((number? exp) 0)
((variable? exp) (if (samevariable? exp var) 1 0))
((sum? exp)
(makesum (deriv (addend exp) var)
(deriv (augend exp) var)))
((product? exp)
(makesum
(makeproduct (multiplier exp)
(deriv (multiplicand exp) var))
(makeproduct (deriv (multiplier exp) var)
(multiplicand exp))))
(else (error "unknown expression type  DERIV" exp))))
We can regard this program as performing a dispatch on the type of
the expression to be differentiated. In this situation the "type
tag" of the datum is the algebraic operator symbol (such as `+')
and the operation being performed is `deriv'. We can transform
this program into datadirected style by rewriting the basic
derivative procedure as
(define (deriv exp var)
(cond ((number? exp) 0)
((variable? exp) (if (samevariable? exp var) 1 0))
(else ((get 'deriv (operator exp)) (operands exp)
var))))
(define (operator exp) (car exp))
(define (operands exp) (cdr exp))
a. Explain what was done above. Why can't we assimilate the
predicates `number?' and `samevariable?' into the
datadirected dispatch?
b. Write the procedures for derivatives of sums and products,
and the auxiliary code required to install them in the table
used by the program above.
c. Choose any additional differentiation rule that you like,
such as the one for exponents (*Note Exercise 256::), and
install it in this datadirected system.
d. In this simple algebraic manipulator the type of an
expression is the algebraic operator that binds it together.
Suppose, however, we indexed the procedures in the opposite
way, so that the dispatch line in `deriv' looked like
((get (operator exp) 'deriv) (operands exp) var)
What corresponding changes to the derivative system are
required?
*Exercise 2.74:* Insatiable Enterprises, Inc., is a highly
decentralized conglomerate company consisting of a large number of
independent divisions located all over the world. The company's
computer facilities have just been interconnected by means of a
clever networkinterfacing scheme that makes the entire network
appear to any user to be a single computer. Insatiable's
president, in her first attempt to exploit the ability of the
network to extract administrative information from division files,
is dismayed to discover that, although all the division files have
been implemented as data structures in Scheme, the particular data
structure used varies from division to division. A meeting of
division managers is hastily called to search for a strategy to
integrate the files that will satisfy headquarters' needs while
preserving the existing autonomy of the divisions.
Show how such a strategy can be implemented with datadirected
programming. As an example, suppose that each division's
personnel records consist of a single file, which contains a set
of records keyed on employees' names. The structure of the set
varies from division to division. Furthermore, each employee's
record is itself a set (structured differently from division to
division) that contains information keyed under identifiers such
as `address' and `salary'. In particular:
a. Implement for headquarters a `getrecord' procedure that
retrieves a specified employee's record from a specified
personnel file. The procedure should be applicable to any
division's file. Explain how the individual divisions' files
should be structured. In particular, what type information
must be supplied?
b. Implement for headquarters a `getsalary' procedure that
returns the salary information from a given employee's record
from any division's personnel file. How should the record be
structured in order to make this operation work?
c. Implement for headquarters a `findemployeerecord'
procedure. This should search all the divisions' files for
the record of a given employee and return the record. Assume
that this procedure takes as arguments an employee's name and
a list of all the divisions' files.
d. When Insatiable takes over a new company, what changes must
be made in order to incorporate the new personnel information
into the central system?
Message passing
...............
The key idea of datadirected programming is to handle generic
operations in programs by dealing explicitly with operationandtype
tables, such as the table in *Note Figure 222::. The style of
programming we used in section *Note 242:: organized the required
dispatching on type by having each operation take care of its own
dispatching. In effect, this decomposes the operationandtype table
into rows, with each generic operation procedure representing a row of
the table.
An alternative implementation strategy is to decompose the table
into columns and, instead of using "intelligent operations" that
dispatch on data types, to work with "intelligent data objects" that
dispatch on operation names. We can do this by arranging things so
that a data object, such as a rectangular number, is represented as a
procedure that takes as input the required operation name and performs
the operation indicated. In such a discipline, `makefromrealimag'
could be written as
(define (makefromrealimag x y)
(define (dispatch op)
(cond ((eq? op 'realpart) x)
((eq? op 'imagpart) y)
((eq? op 'magnitude)
(sqrt (+ (square x) (square y))))
((eq? op 'angle) (atan y x))
(else
(error "Unknown op  MAKEFROMREALIMAG" op))))
dispatch)
The corresponding `applygeneric' procedure, which applies a generic
operation to an argument, now simply feeds the operation's name to the
data object and lets the object do the work:(4)
(define (applygeneric op arg) (arg op))
Note that the value returned by `makefromrealimag' is a
procedurethe internal `dispatch' procedure. This is the procedure
that is invoked when `applygeneric' requests an operation to be
performed.
This style of programming is called "message passing". The name
comes from the image that a data object is an entity that receives the
requested operation name as a "message." We have already seen an
example of message passing in section *Note 213::, where we saw how
`cons', `car', and `cdr' could be defined with no data objects but only
procedures. Here we see that message passing is not a mathematical
trick but a useful technique for organizing systems with generic
operations. In the remainder of this chapter we will continue to use
datadirected programming, rather than message passing, to discuss
generic arithmetic operations. In *Note Chapter 3:: we will return to
message passing, and we will see that it can be a powerful tool for
structuring simulation programs.
*Exercise 2.75:* Implement the constructor `makefrommagang' in
messagepassing style. This procedure should be analogous to the
`makefromrealimag' procedure given above.
*Exercise 2.76:* As a large system with generic operations
evolves, new types of data objects or new operations may be needed.
For each of the three strategiesgeneric operations with explicit
dispatch, datadirected style, and messagepassingstyledescribe
the changes that must be made to a system in order to add new
types or new operations. Which organization would be most
appropriate for a system in which new types must often be added?
Which would be most appropriate for a system in which new
operations must often be added?
 Footnotes 
(1) We use the list `(rectangular)' rather than the symbol
`rectangular' to allow for the possibility of operations with multiple
arguments, not all of the same type.
(2) The type the constructors are installed under needn't be a list
because a constructor is always used to make an object of one
particular type.
(3) `Applygeneric' uses the dottedtail notation described in *Note
Exercise 220::, because different generic operations may take
different numbers of arguments. In `applygeneric', `op' has as its
value the first argument to `applygeneric' and `args' has as its value
a list of the remaining arguments.
`Applygeneric' also uses the primitive procedure `apply', which
takes two arguments, a procedure and a list. `Apply' applies the
procedure, using the elements in the list as arguments. For example,
(apply + (list 1 2 3 4))
returns 10.
(4) One limitation of this organization is it permits only generic
procedures of one argument.
File: sicp.info, Node: 25, Prev: 24, Up: Chapter 2
2.5 Systems with Generic Operations
===================================
In the previous section, we saw how to design systems in which data
objects can be represented in more than one way. The key idea is to
link the code that specifies the data operations to the several
representations by means of generic interface procedures. Now we will
see how to use this same idea not only to define operations that are
generic over different representations but also to define operations
that are generic over different kinds of arguments. We have already
seen several different packages of arithmetic operations: the primitive
arithmetic (`+', `', `*', `/') built into our language, the
rationalnumber arithmetic (`addrat', `subrat', `mulrat', `divrat')
of section *Note 211::, and the complexnumber arithmetic that we
implemented in section *Note 243::. We will now use datadirected
techniques to construct a package of arithmetic operations that
incorporates all the arithmetic packages we have already constructed.
*Note Figure 223:: shows the structure of the system we shall
build. Notice the abstraction barriers. From the perspective of
someone using "numbers," there is a single procedure `add' that
operates on whatever numbers are supplied. `Add' is part of a generic
interface that allows the separate ordinaryarithmetic,
rationalarithmetic, and complexarithmetic packages to be accessed
uniformly by programs that use numbers. Any individual arithmetic
package (such as the complex package) may itself be accessed through
generic procedures (such as `addcomplex') that combine packages
designed for different representations (such as rectangular and polar).
Moreover, the structure of the system is additive, so that one can
design the individual arithmetic packages separately and combine them
to produce a generic arithmetic system.
*Figure 2.23:* Generic arithmetic system.
Programs that use numbers
++
 add sub mul div 
++
Generic arithmetic package
++ ++
 addrat subrat   addcomplex subcomplex  ++
 + + +  * / 
 mulrat divrat    mulcomplex divcomplex   ++
++  ++ 
Rational  Complex artithmetic  Ordinary
arithmetic +++ arithmetic
 Rectangular  Polar 
+++
* Menu:
* 251:: Generic Arithmetic Operations
* 252:: Combining Data of Different Types
* 253:: Example: Symbolic Algebra
File: sicp.info, Node: 251, Next: 252, Prev: 25, Up: 25
2.5.1 Generic Arithmetic Operations

The task of designing generic arithmetic operations is analogous to
that of designing the generic complexnumber operations. We would
like, for instance, to have a generic addition procedure `add' that
acts like ordinary primitive addition `+' on ordinary numbers, like
`addrat' on rational numbers, and like `addcomplex' on complex
numbers. We can implement `add', and the other generic arithmetic
operations, by following the same strategy we used in section *Note
243:: to implement the generic selectors for complex numbers. We
will attach a type tag to each kind of number and cause the generic
procedure to dispatch to an appropriate package according to the data
type of its arguments.
The generic arithmetic procedures are defined as follows:
(define (add x y) (applygeneric 'add x y))
(define (sub x y) (applygeneric 'sub x y))
(define (mul x y) (applygeneric 'mul x y))
(define (div x y) (applygeneric 'div x y))
We begin by installing a package for handling "ordinary" numbers,
that is, the primitive numbers of our language. We will tag these with
the symbol `schemenumber'. The arithmetic operations in this package
are the primitive arithmetic procedures (so there is no need to define
extra procedures to handle the untagged numbers). Since these
operations each take two arguments, they are installed in the table
keyed by the list `(schemenumber schemenumber)':
(define (installschemenumberpackage)
(define (tag x)
(attachtag 'schemenumber x))
(put 'add '(schemenumber schemenumber)
(lambda (x y) (tag (+ x y))))
(put 'sub '(schemenumber schemenumber)
(lambda (x y) (tag ( x y))))
(put 'mul '(schemenumber schemenumber)
(lambda (x y) (tag (* x y))))
(put 'div '(schemenumber schemenumber)
(lambda (x y) (tag (/ x y))))
(put 'make 'schemenumber
(lambda (x) (tag x)))
'done)
Users of the Schemenumber package will create (tagged) ordinary
numbers by means of the procedure:
(define (makeschemenumber n)
((get 'make 'schemenumber) n))
Now that the framework of the generic arithmetic system is in place,
we can readily include new kinds of numbers. Here is a package that
performs rational arithmetic. Notice that, as a benefit of additivity,
we can use without modification the rationalnumber code from section
*Note 211:: as the internal procedures in the package:
(define (installrationalpackage)
;; internal procedures
(define (numer x) (car x))
(define (denom x) (cdr x))
(define (makerat n d)
(let ((g (gcd n d)))
(cons (/ n g) (/ d g))))
(define (addrat x y)
(makerat (+ (* (numer x) (denom y))
(* (numer y) (denom x)))
(* (denom x) (denom y))))
(define (subrat x y)
(makerat ( (* (numer x) (denom y))
(* (numer y) (denom x)))
(* (denom x) (denom y))))
(define (mulrat x y)
(makerat (* (numer x) (numer y))
(* (denom x) (denom y))))
(define (divrat x y)
(makerat (* (numer x) (denom y))
(* (denom x) (numer y))))
;; interface to rest of the system
(define (tag x) (attachtag 'rational x))
(put 'add '(rational rational)
(lambda (x y) (tag (addrat x y))))
(put 'sub '(rational rational)
(lambda (x y) (tag (subrat x y))))
(put 'mul '(rational rational)
(lambda (x y) (tag (mulrat x y))))
(put 'div '(rational rational)
(lambda (x y) (tag (divrat x y))))
(put 'make 'rational
(lambda (n d) (tag (makerat n d))))
'done)
(define (makerational n d)
((get 'make 'rational) n d))
We can install a similar package to handle complex numbers, using
the tag `complex'. In creating the package, we extract from the table
the operations `makefromrealimag' and `makefrommagang' that were
defined by the rectangular and polar packages. Additivity permits us
to use, as the internal operations, the same `addcomplex',
`subcomplex', `mulcomplex', and `divcomplex' procedures from section
*Note 241::.
(define (installcomplexpackage)
;; imported procedures from rectangular and polar packages
(define (makefromrealimag x y)
((get 'makefromrealimag 'rectangular) x y))
(define (makefrommagang r a)
((get 'makefrommagang 'polar) r a))
;; internal procedures
(define (addcomplex z1 z2)
(makefromrealimag (+ (realpart z1) (realpart z2))
(+ (imagpart z1) (imagpart z2))))
(define (subcomplex z1 z2)
(makefromrealimag ( (realpart z1) (realpart z2))
( (imagpart z1) (imagpart z2))))
(define (mulcomplex z1 z2)
(makefrommagang (* (magnitude z1) (magnitude z2))
(+ (angle z1) (angle z2))))
(define (divcomplex z1 z2)
(makefrommagang (/ (magnitude z1) (magnitude z2))
( (angle z1) (angle z2))))
;; interface to rest of the system
(define (tag z) (attachtag 'complex z))
(put 'add '(complex complex)
(lambda (z1 z2) (tag (addcomplex z1 z2))))
(put 'sub '(complex complex)
(lambda (z1 z2) (tag (subcomplex z1 z2))))
(put 'mul '(complex complex)
(lambda (z1 z2) (tag (mulcomplex z1 z2))))
(put 'div '(complex complex)
(lambda (z1 z2) (tag (divcomplex z1 z2))))
(put 'makefromrealimag 'complex
(lambda (x y) (tag (makefromrealimag x y))))
(put 'makefrommagang 'complex
(lambda (r a) (tag (makefrommagang r a))))
'done)
Programs outside the complexnumber package can construct complex
numbers either from real and imaginary parts or from magnitudes and
angles. Notice how the underlying procedures, originally defined in
the rectangular and polar packages, are exported to the complex
package, and exported from there to the outside world.
(define (makecomplexfromrealimag x y)
((get 'makefromrealimag 'complex) x y))
(define (makecomplexfrommagang r a)
((get 'makefrommagang 'complex) r a))
What we have here is a twolevel tag system. A typical complex
number, such as 3 + 4i in rectangular form, would be represented as
shown in *Note Figure 224::. The outer tag (`complex') is used to
direct the number to the complex package. Once within the complex
package, the next tag (`rectangular') is used to direct the number to
the rectangular package. In a large and complicated system there might
be many levels, each interfaced with the next by means of generic
operations. As a data object is passed "downward," the outer tag that
is used to direct it to the appropriate package is stripped off (by
applying `contents') and the next level of tag (if any) becomes visible
to be used for further dispatching.
*Figure 2.24:* Representation of 3 + 4i in rectangular form.
+++ +++ +++
> *  *+> *  *+> *  * 
+++ +++ +++
   
V V V V
++ ++ ++ ++
 complex   rectangular   3   4 
++ ++ ++ ++
In the above packages, we used `addrat', `addcomplex', and the
other arithmetic procedures exactly as originally written. Once these
definitions are internal to different installation procedures, however,
they no longer need names that are distinct from each other: we could
simply name them `add', `sub', `mul', and `div' in both packages.
*Exercise 2.77:* Louis Reasoner tries to evaluate the expression
`(magnitude z)' where `z' is the object shown in *Note Figure
224::. To his surprise, instead of the answer 5 he gets an error
message from `applygeneric', saying there is no method for the
operation `magnitude' on the types `(complex)'. He shows this
interaction to Alyssa P. Hacker, who says "The problem is that the
complexnumber selectors were never defined for `complex' numbers,
just for `polar' and `rectangular' numbers. All you have to do to
make this work is add the following to the `complex' package:"
(put 'realpart '(complex) realpart)
(put 'imagpart '(complex) imagpart)
(put 'magnitude '(complex) magnitude)
(put 'angle '(complex) angle)
Describe in detail why this works. As an example, trace through
all the procedures called in evaluating the expression `(magnitude
z)' where `z' is the object shown in *Note Figure 224::. In
particular, how many times is `applygeneric' invoked? What
procedure is dispatched to in each case?
*Exercise 2.78:* The internal procedures in the `schemenumber'
package are essentially nothing more than calls to the primitive
procedures `+', `', etc. It was not possible to use the
primitives of the language directly because our typetag system
requires that each data object have a type attached to it. In
fact, however, all Lisp implementations do have a type system,
which they use internally. Primitive predicates such as `symbol?'
and `number?' determine whether data objects have particular
types. Modify the definitions of `typetag', `contents', and
`attachtag' from section *Note 242:: so that our generic system
takes advantage of Scheme's internal type system. That is to say,
the system should work as before except that ordinary numbers
should be represented simply as Scheme numbers rather than as
pairs whose `car' is the symbol `schemenumber'.
*Exercise 2.79:* Define a generic equality predicate `equ?' that
tests the equality of two numbers, and install it in the generic
arithmetic package. This operation should work for ordinary
numbers, rational numbers, and complex numbers.
*Exercise 2.80:* Define a generic predicate `=zero?' that tests if
its argument is zero, and install it in the generic arithmetic
package. This operation should work for ordinary numbers, rational
numbers, and complex numbers.
File: sicp.info, Node: 252, Next: 253, Prev: 251, Up: 25
2.5.2 Combining Data of Different Types

We have seen how to define a unified arithmetic system that encompasses
ordinary numbers, complex numbers, rational numbers, and any other type
of number we might decide to invent, but we have ignored an important
issue. The operations we have defined so far treat the different data
types as being completely independent. Thus, there are separate
packages for adding, say, two ordinary numbers, or two complex numbers.
What we have not yet considered is the fact that it is meaningful to
define operations that cross the type boundaries, such as the addition
of a complex number to an ordinary number. We have gone to great pains
to introduce barriers between parts of our programs so that they can be
developed and understood separately. We would like to introduce the
crosstype operations in some carefully controlled way, so that we can
support them without seriously violating our module boundaries.
One way to handle crosstype operations is to design a different
procedure for each possible combination of types for which the
operation is valid. For example, we could extend the complexnumber
package so that it provides a procedure for adding complex numbers to
ordinary numbers and installs this in the table using the tag `(complex
schemenumber)':(1)
;; to be included in the complex package
(define (addcomplextoschemenum z x)
(makefromrealimag (+ (realpart z) x)
(imagpart z)))
(put 'add '(complex schemenumber)
(lambda (z x) (tag (addcomplextoschemenum z x))))
This technique works, but it is cumbersome. With such a system, the
cost of introducing a new type is not just the construction of the
package of procedures for that type but also the construction and
installation of the procedures that implement the crosstype
operations. This can easily be much more code than is needed to define
the operations on the type itself. The method also undermines our
ability to combine separate packages additively, or least to limit the
extent to which the implementors of the individual packages need to
take account of other packages. For instance, in the example above, it
seems reasonable that handling mixed operations on complex numbers and
ordinary numbers should be the responsibility of the complexnumber
package. Combining rational numbers and complex numbers, however,
might be done by the complex package, by the rational package, or by
some third package that uses operations extracted from these two
packages. Formulating coherent policies on the division of
responsibility among packages can be an overwhelming task in designing
systems with many packages and many crosstype operations.
Coercion
........
In the general situation of completely unrelated operations acting on
completely unrelated types, implementing explicit crosstype operations,
cumbersome though it may be, is the best that one can hope for.
Fortunately, we can usually do better by taking advantage of additional
structure that may be latent in our type system. Often the different
data types are not completely independent, and there may be ways by
which objects of one type may be viewed as being of another type. This
process is called "coercion". For example, if we are asked to
arithmetically combine an ordinary number with a complex number, we can
view the ordinary number as a complex number whose imaginary part is
zero. This transforms the problem to that of combining two complex
numbers, which can be handled in the ordinary way by the
complexarithmetic package.
In general, we can implement this idea by designing coercion
procedures that transform an object of one type into an equivalent
object of another type. Here is a typical coercion procedure, which
transforms a given ordinary number to a complex number with that real
part and zero imaginary part:
(define (schemenumber>complex n)
(makecomplexfromrealimag (contents n) 0))
We install these coercion procedures in a special coercion table,
indexed under the names of the two types:
(putcoercion 'schemenumber 'complex schemenumber>complex)
(We assume that there are `putcoercion' and `getcoercion'
procedures available for manipulating this table.) Generally some of
the slots in the table will be empty, because it is not generally
possible to coerce an arbitrary data object of each type into all other
types. For example, there is no way to coerce an arbitrary complex
number to an ordinary number, so there will be no general
`complex>schemenumber' procedure included in the table.
Once the coercion table has been set up, we can handle coercion in a
uniform manner by modifying the `applygeneric' procedure of section
*Note 243::. When asked to apply an operation, we first check
whether the operation is defined for the arguments' types, just as
before. If so, we dispatch to the procedure found in the
operationandtype table. Otherwise, we try coercion. For simplicity,
we consider only the case where there are two arguments.(2) We check
the coercion table to see if objects of the first type can be coerced
to the second type. If so, we coerce the first argument and try the
operation again. If objects of the first type cannot in general be
coerced to the second type, we try the coercion the other way around to
see if there is a way to coerce the second argument to the type of the
first argument. Finally, if there is no known way to coerce either
type to the other type, we give up. Here is the procedure:
(define (applygeneric op . args)
(let ((typetags (map typetag args)))
(let ((proc (get op typetags)))
(if proc
(apply proc (map contents args))
(if (= (length args) 2)
(let ((type1 (car typetags))
(type2 (cadr typetags))
(a1 (car args))
(a2 (cadr args)))
(let ((t1>t2 (getcoercion type1 type2))
(t2>t1 (getcoercion type2 type1)))
(cond (t1>t2
(applygeneric op (t1>t2 a1) a2))
(t2>t1
(applygeneric op a1 (t2>t1 a2)))
(else
(error "No method for these types"
(list op typetags))))))
(error "No method for these types"
(list op typetags)))))))
This coercion scheme has many advantages over the method of defining
explicit crosstype operations, as outlined above. Although we still
need to write coercion procedures to relate the types (possibly n^2
procedures for a system with n types), we need to write only one
procedure for each pair of types rather than a different procedure for
each collection of types and each generic operation.(3) What we are
counting on here is the fact that the appropriate transformation
between types depends only on the types themselves, not on the
operation to be applied.
On the other hand, there may be applications for which our coercion
scheme is not general enough. Even when neither of the objects to be
combined can be converted to the type of the other it may still be
possible to perform the operation by converting both objects to a third
type. In order to deal with such complexity and still preserve
modularity in our programs, it is usually necessary to build systems
that take advantage of still further structure in the relations among
types, as we discuss next.
Hierarchies of types
....................
The coercion scheme presented above relied on the existence of natural
relations between pairs of types. Often there is more "global"
structure in how the different types relate to each other. For
instance, suppose we are building a generic arithmetic system to handle
integers, rational numbers, real numbers, and complex numbers. In such
a system, it is quite natural to regard an integer as a special kind of
rational number, which is in turn a special kind of real number, which
is in turn a special kind of complex number. What we actually have is
a socalled "hierarchy of types", in which, for example, integers are a "subtype"
of rational numbers (i.e., any operation that can be applied to a
rational number can automatically be applied to an integer).
Conversely, we say that rational numbers form a "supertype" of
integers. The particular hierarchy we have here is of a very simple
kind, in which each type has at most one supertype and at most one
subtype. Such a structure, called a "tower", is illustrated in *Note
Figure 225::.
*Figure 2.25:* A tower of types.
complex
^

real
^

rational
^

integer
If we have a tower structure, then we can greatly simplify the
problem of adding a new type to the hierarchy, for we need only specify
how the new type is embedded in the next supertype above it and how it
is the supertype of the type below it. For example, if we want to add
an integer to a complex number, we need not explicitly define a special
coercion procedure `integer>complex'. Instead, we define how an
integer can be transformed into a rational number, how a rational
number is transformed into a real number, and how a real number is
transformed into a complex number. We then allow the system to
transform the integer into a complex number through these steps and
then add the two complex numbers.
We can redesign our `applygeneric' procedure in the following way:
For each type, we need to supply a `raise' procedure, which "raises"
objects of that type one level in the tower. Then when the system is
required to operate on objects of different types it can successively
raise the lower types until all the objects are at the same level in
the tower. (*Note Exercise 283:: and *Note Exercise 284:: concern
the details of implementing such a strategy.)
Another advantage of a tower is that we can easily implement the
notion that every type "inherits" all operations defined on a
supertype. For instance, if we do not supply a special procedure for
finding the real part of an integer, we should nevertheless expect that
`realpart' will be defined for integers by virtue of the fact that
integers are a subtype of complex numbers. In a tower, we can arrange
for this to happen in a uniform way by modifying `applygeneric'. If
the required operation is not directly defined for the type of the
object given, we raise the object to its supertype and try again. We
thus crawl up the tower, transforming our argument as we go, until we
either find a level at which the desired operation can be performed or
hit the top (in which case we give up).
Yet another advantage of a tower over a more general hierarchy is
that it gives us a simple way to "lower" a data object to the simplest
representation. For example, if we add 2 + 3i to 4  3i, it would be
nice to obtain the answer as the integer 6 rather than as the complex
number 6 + 0i. *Note Exercise 285:: discusses a way to implement such
a lowering operation. (The trick is that we need a general way to
distinguish those objects that can be lowered, such as 6 + 0i, from
those that cannot, such as 6 + 2i.)
*Figure 2.26:* Relations among types of geometric figures.
polygon
/ \
/ \
triangle quadrilateral
/ \ / \
/ \ / \
isosceles right trapezoid kite
triangle triangle  
 \   
 \   
equilateral isosceles parallelogram 
triangle right  \ 
triangle  \ 
rectangle rhombus
\ /
\ /
square
Inadequacies of hierarchies
...........................
If the data types in our system can be naturally arranged in a tower,
this greatly simplifies the problems of dealing with generic operations
on different types, as we have seen. Unfortunately, this is usually
not the case. *Note Figure 226:: illustrates a more complex
arrangement of mixed types, this one showing relations among different
types of geometric figures. We see that, in general, a type may have
more than one subtype. Triangles and quadrilaterals, for instance, are
both subtypes of polygons. In addition, a type may have more than one
supertype. For example, an isosceles right triangle may be regarded
either as an isosceles triangle or as a right triangle. This
multiplesupertypes issue is particularly thorny, since it means that
there is no unique way to "raise" a type in the hierarchy. Finding the
"correct" supertype in which to apply an operation to an object may
involve considerable searching through the entire type network on the
part of a procedure such as `applygeneric'. Since there generally are
multiple subtypes for a type, there is a similar problem in coercing a
value "down" the type hierarchy. Dealing with large numbers of
interrelated types while still preserving modularity in the design of
large systems is very difficult, and is an area of much current
research.(4)
*Exercise 2.81:* Louis Reasoner has noticed that `applygeneric'
may try to coerce the arguments to each other's type even if they
already have the same type. Therefore, he reasons, we need to put
procedures in the coercion table to "coerce" arguments of each
type to their own type. For example, in addition to the
`schemenumber>complex' coercion shown above, he would do:
(define (schemenumber>schemenumber n) n)
(define (complex>complex z) z)
(putcoercion 'schemenumber 'schemenumber
schemenumber>schemenumber)
(putcoercion 'complex 'complex complex>complex)
a. With Louis's coercion procedures installed, what happens if
`applygeneric' is called with two arguments of type
`schemenumber' or two arguments of type `complex' for an
operation that is not found in the table for those types?
For example, assume that we've defined a generic
exponentiation operation:
(define (exp x y) (applygeneric 'exp x y))
and have put a procedure for exponentiation in the
Schemenumber package but not in any other package:
;; following added to Schemenumber package
(put 'exp '(schemenumber schemenumber)
(lambda (x y) (tag (expt x y)))) ; using primitive `expt'
What happens if we call `exp' with two complex numbers as
arguments?
b. Is Louis correct that something had to be done about coercion
with arguments of the same type, or does `applygeneric' work
correctly as is?
c. Modify `applygeneric' so that it doesn't try coercion if the
two arguments have the same type.
*Exercise 2.82:* Show how to generalize `applygeneric' to handle
coercion in the general case of multiple arguments. One strategy
is to attempt to coerce all the arguments to the type of the first
argument, then to the type of the second argument, and so on.
Give an example of a situation where this strategy (and likewise
the twoargument version given above) is not sufficiently general.
(Hint: Consider the case where there are some suitable mixedtype
operations present in the table that will not be tried.)
*Exercise 2.83:* Suppose you are designing a generic arithmetic
system for dealing with the tower of types shown in *Note Figure
225::: integer, rational, real, complex. For each type (except
complex), design a procedure that raises objects of that type one
level in the tower. Show how to install a generic `raise'
operation that will work for each type (except complex).
*Exercise 2.84:* Using the `raise' operation of *Note Exercise
283::, modify the `applygeneric' procedure so that it coerces
its arguments to have the same type by the method of successive
raising, as discussed in this section. You will need to devise a
way to test which of two types is higher in the tower. Do this in
a manner that is "compatible" with the rest of the system and will
not lead to problems in adding new levels to the tower.
*Exercise 2.85:* This section mentioned a method for "simplifying"
a data object by lowering it in the tower of types as far as
possible. Design a procedure `drop' that accomplishes this for the
tower described in *Note Exercise 283::. The key is to decide,
in some general way, whether an object can be lowered. For
example, the complex number 1.5 + 0i can be lowered as far as
`real', the complex number 1 + 0i can be lowered as far as
`integer', and the complex number 2 + 3i cannot be lowered at all.
Here is a plan for determining whether an object can be lowered:
Begin by defining a generic operation `project' that "pushes" an
object down in the tower. For example, projecting a complex
number would involve throwing away the imaginary part. Then a
number can be dropped if, when we `project' it and `raise' the
result back to the type we started with, we end up with something
equal to what we started with. Show how to implement this idea in
detail, by writing a `drop' procedure that drops an object as far
as possible. You will need to design the various projection
operations(5) and install `project' as a generic operation in the
system. You will also need to make use of a generic equality
predicate, such as described in *Note Exercise 279::. Finally,
use `drop' to rewrite `applygeneric' from *Note Exercise 284::
so that it "simplifies" its answers.
*Exercise 2.86:* Suppose we want to handle complex numbers whose
real parts, imaginary parts, magnitudes, and angles can be either
ordinary numbers, rational numbers, or other numbers we might wish
to add to the system. Describe and implement the changes to the
system needed to accommodate this. You will have to define
operations such as `sine' and `cosine' that are generic over
ordinary numbers and rational numbers.
 Footnotes 
(1) We also have to supply an almost identical procedure to handle
the types `(schemenumber complex)'.
(2) See *Note Exercise 282:: for generalizations.
(3) If we are clever, we can usually get by with fewer than n^2
coercion procedures. For instance, if we know how to convert from type
1 to type 2 and from type 2 to type 3, then we can use this knowledge to
convert from type 1 to type 3. This can greatly decrease the number of
coercion procedures we need to supply explicitly when we add a new type
to the system. If we are willing to build the required amount of
sophistication into our system, we can have it search the "graph" of
relations among types and automatically generate those coercion
procedures that can be inferred from the ones that are supplied
explicitly.
(4) This statement, which also appears in the first edition of this
book, is just as true now as it was when we wrote it twelve years ago.
Developing a useful, general framework for expressing the relations
among different types of entities (what philosophers call "ontology")
seems intractably difficult. The main difference between the confusion
that existed ten years ago and the confusion that exists now is that
now a variety of inadequate ontological theories have been embodied in
a plethora of correspondingly inadequate programming languages. For
example, much of the complexity of objectoriented programming
languagesand the subtle and confusing differences among contemporary
objectoriented languagescenters on the treatment of generic
operations on interrelated types. Our own discussion of computational
objects in *Note Chapter 3:: avoids these issues entirely. Readers
familiar with objectoriented programming will notice that we have much
to say in *Note Chapter 3:: about local state, but we do not even
mention "classes" or "inheritance." In fact, we suspect that these
problems cannot be adequately addressed in terms of computerlanguage
design alone, without also drawing on work in knowledge representation
and automated reasoning.
(5) A real number can be projected to an integer using the `round'
primitive, which returns the closest integer to its argument.
File: sicp.info, Node: 253, Prev: 252, Up: 25
2.5.3 Example: Symbolic Algebra

The manipulation of symbolic algebraic expressions is a complex process
that illustrates many of the hardest problems that occur in the design
of largescale systems. An algebraic expression, in general, can be
viewed as a hierarchical structure, a tree of operators applied to
operands. We can construct algebraic expressions by starting with a
set of primitive objects, such as constants and variables, and
combining these by means of algebraic operators, such as addition and
multiplication. As in other languages, we form abstractions that
enable us to refer to compound objects in simple terms. Typical
abstractions in symbolic algebra are ideas such as linear combination,
polynomial, rational function, or trigonometric function. We can
regard these as compound "types," which are often useful for directing
the processing of expressions. For example, we could describe the
expression
x^2 sin (y^2 + 1) + r cos 2y + cos(y^3  2y^2)
as a polynomial in x with coefficients that are trigonometric functions
of polynomials in y whose coefficients are integers.
We will not attempt to develop a complete algebraicmanipulation
system here. Such systems are exceedingly complex programs, embodying
deep algebraic knowledge and elegant algorithms. What we will do is
look at a simple but important part of algebraic manipulation: the
arithmetic of polynomials. We will illustrate the kinds of decisions
the designer of such a system faces, and how to apply the ideas of
abstract data and generic operations to help organize this effort.
Arithmetic on polynomials
.........................
Our first task in designing a system for performing arithmetic on
polynomials is to decide just what a polynomial is. Polynomials are
normally defined relative to certain variables (the "indeterminates" of
the polynomial). For simplicity, we will restrict ourselves to
polynomials having just one indeterminate ("univariate
polynomials").(1) We will define a polynomial to be a sum of terms,
each of which is either a coefficient, a power of the indeterminate, or
a product of a coefficient and a power of the indeterminate. A
coefficient is defined as an algebraic expression that is not dependent
upon the indeterminate of the polynomial. For example,
5x^2 + 3r + 7
is a simple polynomial in x, and
(y^2 + 1)r^3 + (2y)x + 1
is a polynomial in x whose coefficients are polynomials in y.
Already we are skirting some thorny issues. Is the first of these
polynomials the same as the polynomial 5y^2 + 3y + 7, or not? A
reasonable answer might be "yes, if we are considering a polynomial
purely as a mathematical function, but no, if we are considering a
polynomial to be a syntactic form." The second polynomial is
algebraically equivalent to a polynomial in y whose coefficients are
polynomials in x. Should our system recognize this, or not?
Furthermore, there are other ways to represent a polynomialfor
example, as a product of factors, or (for a univariate polynomial) as
the set of roots, or as a listing of the values of the polynomial at a
specified set of points.(2) We can finesse these questions by deciding
that in our algebraicmanipulation system a "polynomial" will be a
particular syntactic form, not its underlying mathematical meaning.
Now we must consider how to go about doing arithmetic on
polynomials. In this simple system, we will consider only addition and
multiplication. Moreover, we will insist that two polynomials to be
combined must have the same indeterminate.
We will approach the design of our system by following the familiar
discipline of data abstraction. We will represent polynomials using a
data structure called a "poly", which consists of a variable and a
collection of terms. We assume that we have selectors `variable' and
`termlist' that extract those parts from a poly and a constructor
`makepoly' that assembles a poly from a given variable and a term
list. A variable will be just a symbol, so we can use the
`samevariable?' procedure of section *Note 232:: to compare
variables. The following procedures define addition and multiplication
of polys:
(define (addpoly p1 p2)
(if (samevariable? (variable p1) (variable p2))
(makepoly (variable p1)
(addterms (termlist p1)
(termlist p2)))
(error "Polys not in same var  ADDPOLY"
(list p1 p2))))
(define (mulpoly p1 p2)
(if (samevariable? (variable p1) (variable p2))
(makepoly (variable p1)
(multerms (termlist p1)
(termlist p2)))
(error "Polys not in same var  MULPOLY"
(list p1 p2))))
To incorporate polynomials into our generic arithmetic system, we
need to supply them with type tags. We'll use the tag `polynomial',
and install appropriate operations on tagged polynomials in the
operation table. We'll embed all our code in an installation procedure
for the polynomial package, similar to the ones in section *Note
251:::
(define (installpolynomialpackage)
;; internal procedures
;; representation of poly
(define (makepoly variable termlist)
(cons variable termlist))
(define (variable p) (car p))
(define (termlist p) (cdr p))
<_procedures `samevariable?' and `variable?' from section 2.3.2_>
;; representation of terms and term lists
<_procedures `adjointerm' ... `coeff' from text below_>
;; continued on next page
(define (addpoly p1 p2) ...)
<_procedures used by `addpoly'_>
(define (mulpoly p1 p2) ...)
<_procedures used by `mulpoly'_>
;; interface to rest of the system
(define (tag p) (attachtag 'polynomial p))
(put 'add '(polynomial polynomial)
(lambda (p1 p2) (tag (addpoly p1 p2))))
(put 'mul '(polynomial polynomial)
(lambda (p1 p2) (tag (mulpoly p1 p2))))
(put 'make 'polynomial
(lambda (var terms) (tag (makepoly var terms))))
'done)
Polynomial addition is performed termwise. Terms of the same order
(i.e., with the same power of the indeterminate) must be combined.
This is done by forming a new term of the same order whose coefficient
is the sum of the coefficients of the addends. Terms in one addend for
which there are no terms of the same order in the other addend are
simply accumulated into the sum polynomial being constructed.
In order to manipulate term lists, we will assume that we have a
constructor `theemptytermlist' that returns an empty term list and a
constructor `adjointerm' that adjoins a new term to a term list. We
will also assume that we have a predicate `emptytermlist?' that tells
if a given term list is empty, a selector `firstterm' that extracts
the highestorder term from a term list, and a selector `restterms'
that returns all but the highestorder term. To manipulate terms, we
will suppose that we have a constructor `maketerm' that constructs a
term with given order and coefficient, and selectors `order' and
`coeff' that return, respectively, the order and the coefficient of the
term. These operations allow us to consider both terms and term lists
as data abstractions, whose concrete representations we can worry about
separately.
Here is the procedure that constructs the term list for the sum of
two polynomials:(3)
(define (addterms L1 L2)
(cond ((emptytermlist? L1) L2)
((emptytermlist? L2) L1)
(else
(let ((t1 (firstterm L1)) (t2 (firstterm L2)))
(cond ((> (order t1) (order t2))
(adjointerm
t1 (addterms (restterms L1) L2)))
((< (order t1) (order t2))
(adjointerm
t2 (addterms L1 (restterms L2))))
(else
(adjointerm
(maketerm (order t1)
(add (coeff t1) (coeff t2)))
(addterms (restterms L1)
(restterms L2)))))))))
The most important point to note here is that we used the generic
addition procedure `add' to add together the coefficients of the terms
being combined. This has powerful consequences, as we will see below.
In order to multiply two term lists, we multiply each term of the
first list by all the terms of the other list, repeatedly using
`multermbyallterms', which multiplies a given term by all terms in
a given term list. The resulting term lists (one for each term of the
first list) are accumulated into a sum. Multiplying two terms forms a
term whose order is the sum of the orders of the factors and whose
coefficient is the product of the coefficients of the factors:
(define (multerms L1 L2)
(if (emptytermlist? L1)
(theemptytermlist)
(addterms (multermbyallterms (firstterm L1) L2)
(multerms (restterms L1) L2))))
(define (multermbyallterms t1 L)
(if (emptytermlist? L)
(theemptytermlist)
(let ((t2 (firstterm L)))
(adjointerm
(maketerm (+ (order t1) (order t2))
(mul (coeff t1) (coeff t2)))
(multermbyallterms t1 (restterms L))))))
This is really all there is to polynomial addition and
multiplication. Notice that, since we operate on terms using the
generic procedures `add' and `mul', our polynomial package is
automatically able to handle any type of coefficient that is known
about by the generic arithmetic package. If we include a coercion
mechanism such as one of those discussed in section *Note 252::, then
we also are automatically able to handle operations on polynomials of
different coefficient types, such as
/ 2 \
[3x^2 + (2 + 3i)x + 7] *  x^4 +  x^2 + (5 + 3i) 
\ 3 /
Because we installed the polynomial addition and multiplication
procedures `addpoly' and `mulpoly' in the generic arithmetic system
as the `add' and `mul' operations for type `polynomial', our system is
also automatically able to handle polynomial operations such as
[(y + 1)x^2 + (y^2 + 1)x + (y  1)] * [(y  2)x + (y^3 + 7)]
The reason is that when the system tries to combine coefficients, it
will dispatch through `add' and `mul'. Since the coefficients are
themselves polynomials (in y), these will be combined using `addpoly'
and `mulpoly'. The result is a kind of "datadirected recursion" in
which, for example, a call to `mulpoly' will result in recursive calls
to `mulpoly' in order to multiply the coefficients. If the
coefficients of the coefficients were themselves polynomials (as might
be used to represent polynomials in three variables), the data
direction would ensure that the system would follow through another
level of recursive calls, and so on through as many levels as the
structure of the data dictates.(4)
Representing term lists
.......................
Finally, we must confront the job of implementing a good representation
for term lists. A term list is, in effect, a set of coefficients keyed
by the order of the term. Hence, any of the methods for representing
sets, as discussed in section *Note 233::, can be applied to this
task. On the other hand, our procedures `addterms' and `multerms'
always access term lists sequentially from highest to lowest order.
Thus, we will use some kind of ordered list representation.
How should we structure the list that represents a term list? One
consideration is the "density" of the polynomials we intend to
manipulate. A polynomial is said to be "dense" if it has nonzero
coefficients in terms of most orders. If it has many zero terms it is
said to be "sparse". For example,
A : x^5 + 2x^4 + 3x^2  2x  5
is a dense polynomial, whereas
B : x^100 + 2x^2 + 1
is sparse.
The term lists of dense polynomials are most efficiently represented
as lists of the coefficients. For example, A above would be nicely
represented as `(1 2 0 3 2 5)'. The order of a term in this
representation is the length of the sublist beginning with that term's
coefficient, decremented by 1.(5) This would be a terrible
representation for a sparse polynomial such as B: There would be a
giant list of zeros punctuated by a few lonely nonzero terms. A more
reasonable representation of the term list of a sparse polynomial is as
a list of the nonzero terms, where each term is a list containing the
order of the term and the coefficient for that order. In such a
scheme, polynomial B is efficiently represented as `((100 1) (2 2) (0
1))'. As most polynomial manipulations are performed on sparse
polynomials, we will use this method. We will assume that term lists
are represented as lists of terms, arranged from highestorder to
lowestorder term. Once we have made this decision, implementing the
selectors and constructors for terms and term lists is
straightforward:(6)
(define (adjointerm term termlist)
(if (=zero? (coeff term))
termlist
(cons term termlist)))
(define (theemptytermlist) '())
(define (firstterm termlist) (car termlist))
(define (restterms termlist) (cdr termlist))
(define (emptytermlist? termlist) (null? termlist))
(define (maketerm order coeff) (list order coeff))
(define (order term) (car term))
(define (coeff term) (cadr term))
where `=zero?' is as defined in *Note Exercise 280::. (See also *Note
Exercise 287:: below.)
Users of the polynomial package will create (tagged) polynomials by
means of the procedure:
(define (makepolynomial var terms)
((get 'make 'polynomial) var terms))
*Exercise 2.87:* Install `=zero?' for polynomials in the generic
arithmetic package. This will allow `adjointerm' to work for
polynomials with coefficients that are themselves polynomials.
*Exercise 2.88:* Extend the polynomial system to include
subtraction of polynomials. (Hint: You may find it helpful to
define a generic negation operation.)
*Exercise 2.89:* Define procedures that implement the termlist
representation described above as appropriate for dense
polynomials.
*Exercise 2.90:* Suppose we want to have a polynomial system that
is efficient for both sparse and dense polynomials. One way to do
this is to allow both kinds of termlist representations in our
system. The situation is analogous to the complexnumber example
of section *Note 24::, where we allowed both rectangular and
polar representations. To do this we must distinguish different
types of term lists and make the operations on term lists generic.
Redesign the polynomial system to implement this generalization.
This is a major effort, not a local change.
*Exercise 2.91:* A univariate polynomial can be divided by another
one to produce a polynomial quotient and a polynomial remainder.
For example,
x^5  1
 = x^3 + x, remainder x  1
x^2  1
Division can be performed via long division. That is, divide the
highestorder term of the dividend by the highestorder term of
the divisor. The result is the first term of the quotient. Next,
multiply the result by the divisor, subtract that from the
dividend, and produce the rest of the answer by recursively
dividing the difference by the divisor. Stop when the order of the
divisor exceeds the order of the dividend and declare the dividend
to be the remainder. Also, if the dividend ever becomes zero,
return zero as both quotient and remainder.
We can design a `divpoly' procedure on the model of `addpoly' and
`mulpoly'. The procedure checks to see if the two polys have the
same variable. If so, `divpoly' strips off the variable and
passes the problem to `divterms', which performs the division
operation on term lists. `Divpoly' finally reattaches the
variable to the result supplied by `divterms'. It is convenient
to design `divterms' to compute both the quotient and the
remainder of a division. `Divterms' can take two term lists as
arguments and return a list of the quotient term list and the
remainder term list.
Complete the following definition of `divterms' by filling in the
missing expressions. Use this to implement `divpoly', which
takes two polys as arguments and returns a list of the quotient
and remainder polys.
(define (divterms L1 L2)
(if (emptytermlist? L1)
(list (theemptytermlist) (theemptytermlist))
(let ((t1 (firstterm L1))
(t2 (firstterm L2)))
(if (> (order t2) (order t1))
(list (theemptytermlist) L1)
(let ((newc (div (coeff t1) (coeff t2)))
(newo ( (order t1) (order t2))))
(let ((restofresult
))