Fundamentals of CS I (CS151 2001S)

Variable-Arity Procedures


Exercise 0: Preparation

a. Please scan through the reading on variable arity procedures.

b. Start DrScheme.

Exercise 1: Experiments with display-line

Here is the display-line procedure from the reading.

;;; Procedure:
;;;   display-line
;;; Parameters:
;;;   0 or more values
;;; Purpose:
;;;   Displays the strings terminated by a carriage return.
;;; Produces:
;;;   Nothing
;;; Preconditions:
;;;   (none)
;;; Postconditions:
;;;   The standards
(define display-line
  (lambda arguments
    (let kernel ((rest arguments))
      (if (null? rest)
            (display (car rest))
            (kernel (cdr rest)))))))

a. Try out some other calls to display-line to check what it prints. For example, try the following:

(display-line "going" "going" "gone")
(display-line "countdown:" 5 4 3 2 1 "done")
(display-line)          ;; apply display-line to no arguments

b. Explain your results.

Exercise 2: Extending display-line

The current version of display-line prints all text together without spaces. Modify the code, so that one space is printed between any two adjacent values supplied as arguments to display-line. For instance, after your modifications, the example from the reading will change. It will now be ...

> (display-line "+--" "Here is a string!" "--+")
+-- Here is a string! --+

You may not use display-separated-line in your answer to this question.

Exercise 3: Determining the number of arguments

Define and test a procedure named call-arity that takes any number of arguments and returns the number of arguments it received (ignoring their values):

> (call-arity 'a #\b "c" '(d))
> (call-arity 0.0)
> (call-arity)

Exercise 4: Experiments with display-separated-line

a. What happens if you invoke display-separated-line without giving it any arguments?

b. What happens when you give it only one argument?

c. What happens when you give it two arguments?

c. What happens when you give it three arguments?

Exercise 5: A Clicker

Define and test a procedure clicker that takes one or more arguments, of which the first must be an integer and each of the others must be either the symbol 'up or the symbol 'down. Clicker should start from the given integer, add 1 for each 'up argument, subtract 1 for each 'down argument, and return the result:

> (clicker 17 'up 'up)
> (clicker -12 'down 'up 'down 'down 'down)
> (clicker 100)

Exercise 6: Multiple Separators

In writing, we often separate the last element of a list using a different separator than for the prior elements. For example, we might separate the all but the last element with commas and the last element with "and". Extend display-separated-line so that it requires two parameters (the default separator and the final separator) and supports as many the client provides.



Tuesday, 31 October 2000 [Samuel A. Rebelsky]

Thursday, 12 April 2001 [Samuel A. Rebelsky]


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