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Summary: Many of Scheme's control structures, such as
conditionals (which you'll
learn about subsequently), need mechanisms
for constructing tests that return true of false. These tests are also
useful for gathering information about values. In this reading,
we consider Scheme's structures that support such tests.
Contents:
A Boolean value is a datum that reflects the outcome of a
single yes-or-no test. For instance, if one were to ask Scheme to
compute whether the empty list has five elements, it would be able
to determine that it does not, and it would signal this result
by displaying the Boolean value for no
or false
,
which is #f. There is only one other Boolean value,
the one meaning yes
or true
, which is #t.
These are called Boolean values
in honor of the logician George
Boole, who was the first to develop a satisfactory formal theory
of them. (Some folks now talk about fuzzy logic
that includes
values other than true
and false
, but that's beyond the
scope of this course.)
A predicate is a procedure that always returns a Boolean value.
A procedure call in which the procedure is a predicate performs
some yes-or-no test on its arguments. For instance, the predicate
number? (the question mark is part of the name of the
procedure) takes one argument and returns #t if that argument
is a number, #f if it does not. Similarly, the predicate
even? takes one argument, which must be an integer, and
returns #t if the integer is even and #f if it
is odd. The names of most Scheme predicates end with question marks,
and Grinnell's computer scientists recommend this useful convention,
even though it is not required by the rules of the programming language.
(If you ever notice that I've failed to include a question mark in a
predicate and you're the first to tell me, I'll give you some extra
credit.)
Scheme provides a few predicates that let you test the type
of value you're working with.
-
number? tests
whether its argument is a number.
-
symbol? tests whether
its argument is a symbol.
-
string? tests whether
its argument is a string.
-
procedure? tests
whether its argument is a procedure.
-
boolean? tests
whether its argument is a Boolean value.
-
list? tests whether
its argument is a list. (Warning! It can be quite expensive
to determine whether or not something is a list.)
Scheme provides one other basic predicate for working with lists.
-
null? tests whether its
argument is the empty list
value.
Scheme provides a variety of predicates for testing equality.
-
eq? tests whether
its two arguments are identical, in the very narrow sense of occupying
the same storage location in the computer's memory. In practice, this
is useful information only if at least one argument is known to be a
symbol, a Boolean value, or an integer.
-
eqv? tests whether
its two arguments should normally be
regarded as the same object
(as the language standard declares). Note,
however, that two lists can have the same elements without being regarded
as the same object
. Also note that in Scheme's view the number 5, which
is exact
, is not necessarily the same object as the number 5.0, which
might be an approximation.
-
equal? tests whether
its two arguments are the same or,
in the case of lists, whether they have the same contents.
-
= tests whether its arguments,
which must all be
numbers, are numerically equal; 5 and 5.0 are numerically equal for this
purpose.
For this class, you are not required to understand the difference between
the eq? and eqv? procedures. In particular, you
need not plan to use the eqv? procedure. At least for the first
half of the semester, you also need not understand the difference between
the eq? and equal? procedures. Feel free to
use equal? almost exclusively, except when dealing with numbers,
in which case you should use =.
Scheme also provides many numeric predicates, some of which you may have
already explored.
-
< tests whether its arguments, which must all be
numbers, are in strictly ascending numerical order. (The
< operation is one of the few built-in predicates
that does not have an accompanying question mark.)
-
> tests whether its arguments,
which must all be numbers, are in strictly descending numerical order.
-
<= tests whether its arguments, which must all be
numbers, are in ascending numerical order, allowing equality.
-
>= tests whether its arguments, which must all be
numbers, are in descending numerical order, allowing equality.
-
even? tests whether its
argument, which must be an integer, is even.
-
odd? tests whether its argument,
which must be an integer, is odd.
-
zero? tests whether its argument, which must be a number, is equal to zero.
-
positive? tests whether
its argument, which must be a real number, is positive.
-
negative? tests whether
its argument, which must be a real number, is negative.
Another useful Boolean procedure is not, which takes one
argument and returns #t if the argument is #f
and #f if the argument is anything else. For example,
one can test whether the square root of 100 is unequal to the absolute
value of negative twelve by giving the command
(not (= (sqrt 100) (abs -12)))
If Scheme says that the value of this expression is #t, then
the two numbers are indeed unequal.
Two other useful Boolean operations are and and or.
Can you guess what they do?
The and and or keywords have simple logical
meanings. In particular, the and of a collection of Boolean
values is true if all are true and false if any value is false, the
or of a collection of Boolean values is true if any of the
values is true and false if all the values are false. For example,
> (and #t #t #t)
#t
> (and (< 1 2) (< 2 3))
#t
> (and (odd? 1) (odd? 3) (odd? 5) (odd? 6))
#f
> (and)
#t
> (or (odd? 1) (odd? 3) (odd? 5) (odd? 6))
#t
> (or (even? 1) (even? 3) (even? 4) (even? 5))
#t
> (or)
#f
But and and or can be used for so much more.
In fact, they can be used as control structures.
In an and-expression, the expressions that follow the
keyword and are evaluated in succession until one is
found to have the value #f (in which case the rest of the
expressions are skipped and the #f becomes the value of
the entire and-expression). If, after evaluating all of
the expressions, none is found to be #f then the value of
the last expression becomes the value of the entire and
expression. This evaluation strategy gives the programmer a way to
combine several tests into one that will succeed only if all of its
parts succeed.
This strategy also gives the programmer a way to avoid meaningless
tests. For example, we should not make the comparison (< a b)
unless we are sure that both a and b are numbers.
In an or expression, the expressions that follow the
keyword or are evaluated in succession until one is found
to have a value other than#f, in which case the rest of the
expressions are skipped and this value becomes the value of the entire
or-expression. If all of the expressions have been evaluated
and all have the value #f, then
the value of the or-expression is #f.
This gives the programmer a way to combine several tests into one that
will succeed if any of its parts succeeds.
In these cases, and returns the last parameter it encounters
(or false, if it encounters a false value) while or returns
the first non-false value it encounters. For example,
> (and 1 2 3)
3
> (define x 'two)
> (define y 3)
> (+ x y)
+: expects type <number> as 1st argument, given: two; other arguments were: 3
> (and (number? x) (number? y) (+ x y))
#f
> (define x 2)
> (and (number? x) (number? y) (+ x y))
5
> (or 1 2 3)
1
> (or 1 #f 3)
1
> (or #f 2 3)
2
> (or #f #f 3)
3
We can use the ideas above to make an addition procedure that returns
#f if either parameter is not a number. We might say that
such a procedure is a bit safer than the normal addition procedure.
;;; Procedure:
;;; safe-add
;;; Parameters:
;;; x, a number [verified]
;;; y, a number [verified]
;;; Purpose:
;;; Add x and y.
;;; Produces:
;;; sum, a number.
;;; Preconditions:
;;; (No additional preconditions)
;;; Postconditions:
;;; sum = x + y
;;; Problems:
;;; If either x or y is not a number, sum is #f.
(define safe-add
(lambda (x y)
(and (number? x) (number? y) (+ x y))))
Let's compare this version to the standard addition procedure, +.
> (+ 2 3)
5
> (safe-add 2 3)
5
> (+ 2 'three)
+: expects type <number> as 2nd argument, given: three; other arguments were: 2
> (safe-add 2 'three)
#f
If we'd prefer to return 0, rather than #f, we could add an
or clause.
(define safer-add
(lambda (x y)
(or (and (number? x) (number? y) (+ x y))
0)))
In most cases, safer-add acts much like safe-add.
However, when we use the result of the two procedures as an argument to
another procedure, we get a little bit further through the calculation.
> (* 4 (+ 2 3))
20
> (* 4 (safer-add 2 3))
20
> (* 4 (+ 2 'three))
+: expects type <number> as 2nd argument, given: three; other arguments were: 2
> (* 4 (safe-add 2 'three))
*: expects type <number> as 2nd argument, given: #f; other arguments were: 4
> (* 4 (safer-add 2 'three))
0
Different situations will call for different choices between those strategies.
You may note that we were careful to describe and and
or as keywords
rather than as procedures
.
The distinction is an important one. Although keywords look remarkably
like procedures, Scheme distinguishes keywords from procedures by the
order of evaluation of the parameters. For procedures, all the parameters
are evaluated and then the procedure is applied. For keywords, not
all parameters need be evaluated, and particular orders of evaluation
are possible.
If and and or were procedures, we could not
guarantee their control behavior. We'd also get some ugly errors. For
example, consider the revised definition of even? below:
(define new-even?
(lambda (val)
(and (integer? val) (even? val))))
Suppose new-even? is called with 2/3 as a parameter.
In the keyword implementation of and, the first test,
(integer? 2/3), fails, and new-even? returns
false. If and were a procedure, we would still evaluate
the (even? val), and that test would generate an error.
Although most computer scientists, philosophers, and mathematicians prefer
the purity of dividing the world into true
and false
, Scheme
supports a somewhat more general separation. In Scheme, anything that is
not false is considered true. Hence, you can use expressions that return
values other than truth values wherever a truth value is expected. For
example,
> (and #t 1)
1
> (or 3 #t #t)
3
> (not 1)
#f
> (not (not 1))
#t
;
