Computer Science Fundamentals (CS153 2004S)
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In this lab, you will have the opportunity to explore a number of issues relating to predicates, Boolean values, and conditional operations.
Procedures covered in this lab include:
boolean?
,
integer?
,
list?
,
null?
,
number?
,
pair?
,
procedure?
,
symbol?
=
,
eq?
,
eqv?
,
equal?
,
<
(strictly less than),
<=
(less than or equal to),
=
(equal to),
>=
(greater than or equal to),
>
(strictly greater than)
and
,
or
,
not
not
?
and
and or
You may find it helpful to rescan the readings on Boolean values and conditionals.
You may also want to rescan the reading on numbers.
After making sure that you're prepared, start DrScheme.
The null?
predicate determines whether its parameter
is an empty list.
Which of the following does Scheme consider an empty list?
null
'null
()
(list a)
(list)
'nothing
"()"
Consider the following definitions
(define alpha (list 'a 'b 'c)) (define beta (list 'a 'b 'c)) (define gamma alpha)
Determine which of the lists are
eq?
,
eqv?
,
or
equal?
.
What, if anything, did you find surprising in the results of the previous exercises?
By looking at
the Scheme report, see if you can find a pair of values
that are equal in the sense of =
but not in the sense of eqv
.
not
?
a. What type is not
?
b. What predicate would you use to verify your answer?
a. Write a Boolean expression that determines if the value named by
grade
is between 0 and 100, inclusive.
b. Test that expression using different values of grade
.
and
and or
a. Determine the value
and
returns when called with
no parameters.
b. Explain why you think the designers of Scheme had
and
return that value.
c. Determine the value or
returns when called with
no parameters.
d. Explain why you think the designers of Scheme had
or
return that value.
Define and test a Scheme predicate symbolorlist?
that
takes one argument and returns #t
if the argument is either a
symbol or a list, #f
if it is neither.
Define and test a Scheme predicate between?
that takes three
arguments, all real numbers, and determines whether the second one lies
strictly between the first and third (returning #t
if it is,
#f
if it is not). For example, 6 lies strictly between 5 and
13, so both (between? 5 6 13)
and
(between? 13 6 5)
should have the value
#t
.
Three line segments can be assembled into a triangle if, and only
if, the length of each of them is less than the sum of the lengths of the
other two. Define a Scheme predicate triangle?
that takes
three arguments, all positive real numbers, and determines whether line
segments of those three lengths (assumed to be measured in the same units)
could be assembled into a triangle.
Define and test a Scheme procedure neighbor
that takes one
argument, an integer, and returns the next higher integer if its argument
is even, the next lower integer if its argument is odd. (Start by writing
a comment that describes the purpose of the procedure.)
For each of the following expressions, guess what the output should be and then test it in Scheme.
a. (if #t 'aardvark 'zebra)
b. (if #f 'aardvark 'zebra)
c. (if (null? null) 'aardvark 'zebra)
d. (if (null? 'null) 'aardvark 'zebra)
e. (if (null? (list 'a 'b 'c)) 'aardvark 'zebra)
f. (if () 'aardvark 'zebra)
g. (if (list 'a 'b 'c) 'aardvark 'zebra)
h. (if 2 'aardvark 'zebra)
i. (if 'true 'aardvark 'zebra)
j. (if 'false 'aardvark 'zebra)
As you may know, one of the famous riddles of the Sphinx goes something like the following:
What is it that walks upon four legs, then two legs, then three legs?
The answer is humans
.
Write a Scheme procedure, legs
, that, given someone's age, tells
how many legs they walk upon. (You get to choose reasonable ages for the
three phases of life.)
(and)
has value true because
. Since it has no parameters,
none are false.
and
has a value
of true if
none of the parameters have value false
(or)
has value false because
. Since it has no parameters,
none are nonfalse.
or
has value false
if none of the parameters is nonfalse
Wednesday, 6 September 2000 [Samuel A. Rebelsky]
Friday, 8 September 2000 [Samuel A. Rebelsky]
Wednesday, 31 January 2001 [Samuel A. Rebelsky]
and
and or
with no parameters.
Thursday, 1 February 2001 [Samuel A. Rebelsky]
and
and or
with no parameters.
Wednesday, 7 February 2001 [Samuel A. Rebelsky]
Tuesday, 10 September 2002 [Samuel A. Rebelsky]
http://www.cs.grinnell.edu/~rebelsky/Courses/CS151//2002F/Labs/conditionals.html
.
Tuesday, 28 January 2003 [Samuel A. Rebelsky]
http://www.cs.grinnell.edu/~rebelsky/Courses/CS153//2003S/Labs/conditionals.html
.
Thursday, 28 Janaury 2003 [Samuel A. Rebelsky]
http://www.cs.grinnell.edu/~rebelsky/Courses/CS153//2004S/Labs/conditionals.html
.
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[CS153 2003S]
[CS151 2003F]
[CS152 2000F]
[SamR]
Disclaimer:
I usually create these pages on the fly
, which means that I rarely
proofread them and they may contain bad grammar and incorrect details.
It also means that I tend to update them regularly (see the history for
more details). Feel free to contact me with any suggestions for changes.
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.
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Samuel A. Rebelsky, rebelsky@grinnell.edu