Laboratory Exercises For Computer Science 151

Conditional Execution in Scheme

Goals: This laboratory exercise helps you gain some experience developing and using Boolean expressions.

Summary: The Scheme language includes the expressions "and", "or", and "not". When these are applied to Boolean values ( #t and #f), the results follow the conventions from mathematics. Thus, (and A B) is true if both A and B are true and false otherwise. (or A B) is true (#t) if either A is true or B is true (or both). not applied to any true value produces false (#f), and (not) applied to false (#f) returns true (#t).

More generally, "and" and "or" may take as many arguments as desired. "and" is true if all arguments are true, and "or" is true if any of the arguments is true.

As the text notes, Scheme is reasonably liberal in its evaluation of expressions, in that anything that is not explicitly false (#f) is considered true.

Scheme's cond statement uses expressions to determine what action is to be taken. This is illustrated in the following procedure:


    (define type-of-number
        (lambda (A)
            (cond ((< A 0) "the number is negative")
                  ((> A 0) "the number is positive")
                  (else    "the number is zero")
            )
        )
    )

Within this cond expression, (< A 0) is first examined. If it is true, the clause
"the number is negative" is evaluated and returned. [Evaluation of the cond expression is finished.] If, however, the expression (< A 0) is false, then evaluation proceeds with the next expression (> A 0). Again, if this expression is true, the following expression
"the number is positive" is evaluated, and evaluation of cond is completed. But if this expression also is false, then we move to the next part of cond. else signifies that if we get this far, then the action that follows should be evaluated.

Scheme's if expression uses a conditional expression to determine which of two actions to take. This is illustrated in the following procedure:


    (define is-negative
        (lambda (A)
            (if (< A 0) 
                    "the number is negative"
                    "the number is nonnegative"
            )
        )
    )

Steps for this Lab:

Evaluate the following expressions:


   (and #t #t)
   (and #f #t)
   (and #t #f)
   (and #f #f)
   (and #t #t #t #t)
   (and #t #t #f #t)
   (or #t #t)
   (or #f #t)
   (or #t #f)
   (or #f #f)
   (or #t #t #t #t)
   (or #f #t #f #f)
   (or #f #f #f #f)
   (not #t)
   (not #f)

In each case, explain why Scheme produced the result given.

Now, evaluate the following expressions:


   (and 'cat #t)
   (and #f 'cat)
   (and 'cat #f)
   (and #f #f)
   (and #t #t 'cat #t)
   (and #t #t #t 'cat)
   (and #t #t #t 'cat 'dog)
   (or #t 'cat)
   (or 'cat #t)
   (or #f 'cat)
   (or 'cat #f)
   (or 'cat 'dog)
   (or #f #f)
   (not 'cat)

In each case, state why you think Scheme produces the result given. Generalize the results to indicate what values and, or, and not return when they are applied to non-Boolean values.

Consider the following Scheme procedure:
```
   (define not-and
       (lambda (A B)
           (not (and A B))))
```
Explain why this procedure evaluates the logical expression "Not (A And B))".
Apply this procedure to various values #t, #f) for A and B. In each case, be sure you understand why the machine prints the output that results.
Write procedures to evaluate "(A And B) Or C" and "A And (B Or C)".
Run these procedures with various values (#t, #f) for A, B, and C, and examine the output. In each case, be sure you can explain the resulting output.
Enter type-of-number as defined above into Scheme, and apply it to several numbers. In each case, explain how Scheme obtained its answer.
Change the previous procedure to compare2 which is applied to two numbers A and B. If A < B , the procedure should return "The first number is smaller than the second.". If B is smaller, the procedure should return "The first number is greater than the second." If both numbers have the same value, the procedure should return "The numbers are equal."
Program ~walker/151s/labs/smallest.ss contains the shell of a program that is supposed to identify the smallest of three numbers.
- Copy ~walker/151s/labs/smallest.ss to your account.
  This may be done with the copy or cp command as follows:
```
    cp  ~walker/151s/labs/smallest.ss  .
```
  This command should be executed in an dtterm window. Within this command, the final dot . is an abbreviation for your current directory. Thus, in this expression, you are telling the machine to copy (cp) the current file ~walker/151s/labs/smallest.ss to your current directory. The new file will appear in your directory with the same name, smallest.ss. If you wanted the program to have a different name (e.g., my-program.ss), then you could change your copy command to:
```
    cp  ~walker/151s/labs/smallest.ss  my-program.ss
```
- Fill in the Boolean conditions (indicated as ??? in the file) to test for the cases specified.
- Test your procedure with several examples which cover each case.

Rewrite your procedure smallest, replacing the cond by if expressions. Thus, the main part of your procedure may have the form:


    (if (???1) 
        "A is smaller than both B and C." #then clause
        (;;; the else clause -- A is not smaller 
         if (???2) #second condition within cond
             "B is smaller than both A and C."
             ;;; the second else cause -- B is not smaller either
             ??? etc.
        )
    )

Textbook Exercises: As you have time, work on exercises 2.6-2.9 from the textbook.
Challenge Problem 1: Program ~walker/151s/labs/triangle.ss contains the shell of a program that is supposed to determine if a triangle can be made using segments of three specified sides. If such a triangle can be formed, then the program also is supposed to identify the type of triangle.
Copy this program to your account, and fill in the Boolean conditions.
Challenge Problem 2: Rewrite your procedure triangle, using nested if expressions instead of cond.

This document is available on the World Wide Web as

http://www.math.grin.edu/~walker/courses/151.sp99/lab-cond-if.html

created February 2, 1997
last revised February 6, 1999