Fundamentals of Computer Science I: Media Computing (CS151.01 2008S)
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Related Courses: [CSC151.02 2008S (Davis)] [CSC151 2007F (Rebelsky)] [CSC151 2007S (Rebelsky)] [CSCS151 2005S (Stone)]
Summary: Many of Scheme's control structures, such as conditionals (which you'll learn about in a subsequent reading), need mechanisms for constructing tests that return true or false. These tests can also be useful for gathering information about values. In this reading, we consider the types, basic procedures, and mechanisms for combining results that support such tests.
When writing complex programs, we often need to ask questions about the values with which we are computing. Is this pixel a shade of red? Is this image at least 100x100? Are these two colors close enough to be indistinguishable? Is this a light or dark color? Most frequently, these questions (which we often phrase as tests) are used in control structures. For example, we might decide to do one thing for large images and another for small images or we might replace light colors by white and dark colors by black.
To express these kinds of questions, we need a variety of tools. First, we need a type in which to express the valid answers to questions. Second, we need a collection of procedures that can answer simple questions. Third, we need ways to combine questions. Finally, we need control structures that use these questions. In the subsequent sections of this reading, we consider each of these issues. We return to more complex control structures in a subsequent reading.
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 pure red has a high blue component,
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 we've failed to
include a question mark in a predicate and you're the first to tell me,
we'll give you some extra credit.)
Scheme provides a wide variety of basic predicates and DrFu adds a few more. We will consider a few right now, but learn more as the course progresses.
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.
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.
DrFu adds a few special predicates that are tailored to working with colors and images.
image?
tests
whether its argument can be interpreted as an image.
rgb?
tests whether its argument can be
interpreted as an RGB color.
cname?
tests whether its argument can be
interpreted as a color name.
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.
not
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.
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
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 custom 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 extended version of the even?
predicate 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, (
,
fails, and integer?
2.3)new-even?
returns false. If
and
were a procedure, we would still evaluate
the (
, and that test would
generate an error, since even?
val)even?
can only be called
on integers.
Although many 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 “truish”. 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
Can we write predicates that work with colors? Certainly. One simple question is whether we might consider two colors near to each other. What are criteria for making that decision? One possibility is that we will consider two colors similar if all of their components are within 8 of each other. We can define that predicate as follows:
;;; Procedure: ;;; colors-similar? ;;; Parameters: ;;; color1, an RGB color ;;; color2, an RGB color ;;; Purpose: ;;; Determines if color1 and color2 are similar. ;;; Produces: ;;; similar?, a Boolean value ;;; Preconditions: ;;; [No additional] ;;; Postconditions: ;;; If color1 and color2 are close enough to be considered similar, ;;; then similar? is #t. ;;; Otherwise, similar? is #f. ;;; We use a proprietary technique to decide what "close enough" means. (define colors-similar? (lambda (color1 color2) (and (>= 8 (abs (- (rgb-red color1) (rgb-red color2)))) (>= 8 (abs (- (rgb-green color1) (rgb-green color2)))) (>= 8 (abs (- (rgb-blue color1) (rgb-bluecolor2)))))))
Here's a pair of useful predicates: One computes whether a color might reasonably be considered light; another computes whether a color might reasonably consider dark.
;;; Procedure: ;;; rgb-light? ;;; Parameters: ;;; color, an RGB color ;;; Purpose: ;;; Determine if the color seems light. ;;; Produces: ;;; light?, a Boolean value ;;; Preconditions: ;;; [None] ;;; Postconditions: ;;; light? is true (#t) if color's intensity is relatively high. ;;; light? is false (#f) otherwise. (define rgb-light? (lambda (color) (<= 192 (+ (* 0.30 (rgb-red color)) (* 0.59 (rgb-green color)) (* 0.11 (rgb-blue color)))))) ;;; Procedure: ;;; rgb-dark? ;;; Parameters: ;;; color, an RGB color ;;; Purpose: ;;; Determine if the color seems dark. ;;; Produces: ;;; dark?, a Boolean value ;;; Preconditions: ;;; [None] ;;; Postconditions: ;;; dark? is true (#t) if color's intensity is relatively low. ;;; dark? is false (#f) otherwise. (define rgb-dark? (lambda (color) (>= 64 (+ (* 0.30 (rgb-red color)) (* 0.59 (rgb-green color)) (* 0.11 (rgb-blue color))))))
We've seen how and
and or
can be used to combine tests. 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
(
unless we are sure that
both <
a b)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)
Error: +: argument 2 must be: number>
(safe-add 2 'three)
#f
If we'd prefer to return 0, rather than #f
, we could add an
or
clause.
;;; Procedure: ;;; safer-add ;;; Parameters: ;;; x, a number [verified] ;;; y, a number [verified] ;;; Purpose: ;;; Add x and y. ;;; Produces: ;;; sum, a number. ;;; Preconditions: ;;; [No additional preconditions] ;;; Postconditions: ;;; If both x and y are numbers, sum = x + y ;;; Problems: ;;; If either x or y is not a number, sum is 0. (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))
Error: +: argument 2 must be: number>
(* 4 (safe-add 2 'three))
Error: *: argument 2 must be: number>
(* 4 (safer-add 2 'three))
0
Different situations will call for different choices between those strategies.
Here's a simple application of the preceding strategies: We can write a procedure that, given a color, returns black if the color is dark, white if the color is light, and grey if the color is neither dark nor light.
How? Well, we can use and
to compute either black,
if the color is dark, or #f
, if the color is not dark.
(and (rgb-dark? color) color-black)
Similarly, we can use and
to compute either white,
if the color is light, or #f
if the color is not light.
(and (rgb-light? color) color-white)
Finally, we can use or
to put it all together.
;;; Procedure: ;;; rgb-bgw ;;; Parameters: ;;; color, an RGB color ;;; Purpose: ;;; Convert an RGB color to black, grey, or white, depending on ;;; the intensity of the color. ;;; Produces: ;;; bgw, an RGB color ;;; Preconditions: ;;; rgb-light? and rgb-dark? are defined. ;;; Postconditions: ;;; If (rgb-light? color) and not (rgb-dark? color), then bgw is white. ;;; If (rgb-dark? color) and not (rgb-light? color), then bgw is black. ;;; If neither (rgb-light? color) nor (rgb-dark? color), then bgw is ;;; grey. ;;; Problems: ;;; In the unexpected case that none of the above conditions holds, ;; bgw will be one of black, white, and grey. (define rgb-bgw (lambda (color) (or (and (rgb-light? color) color-white) (and (rgb-dark? color) color-black) color-grey)))
Primary: [Front Door] [Syllabus] - [Academic Honesty] [Instructions]
Current: [Outline] [EBoard] [Reading] [Lab] [Assignment]
Groupings: [Assignments] [EBoards] [Examples] [Exams] [Handouts] [Labs] [Outlines] [Projects] [Readings]
References: [A-Z] [Primary] [Scheme Report (R5RS)] [Scheme Reference] [DrScheme Manual]
Related Courses: [CSC151.02 2008S (Davis)] [CSC151 2007F (Rebelsky)] [CSC151 2007S (Rebelsky)] [CSCS151 2005S (Stone)]
Copyright (c) 2007-8 Janet Davis, Matthew Kluber, and Samuel A. Rebelsky. (Selected materials copyright by John David Stone and Henry Walker and used by permission.)
This material is based upon work partially supported by the National Science Foundation under Grant No. CCLI-0633090. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
This work is licensed under a Creative Commons
Attribution-NonCommercial 2.5 License. To view a copy of this
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