Lab: Writing your own procedures

Assigned

Friday, 3 September 2021

Summary

We explore techniques for writing proceres in Racket.

Syntax to remember

• Defining names: (define <name> <expression>).
• Function expressions: (lambda (<parameters>) <expression>).
• Defining functions: (define <name> (lambda (<parameters>) <expression>))

The lab

As in the previous lab (and future labs), you will work with a randomly assigned partner using a starter file.

• procedures.rkt

Remember to employ good pair programming practices as discussed in the previous lab!

• One person, the driver, will share their screen via MS Teams and is in control of keyboard. They focus on the immediate task of designing and coding a solution, speaking aloud about their design thoughts as they work.
• The other person, the navigator, acts as a reviewer. They observe and think more about strategic architectural issues. They look for potential issues and raise them with the driver. They are also responsible for keeping track of the time spent on the problem.

Work

All of the instructions are in the file. Switch to that now.

Additional practice

If you find that you complete the laboratory with time to spare, you might consider doing one or more of the following exercises.

Snowmen, revisited

As you may have noted, your colorful-snowman procedure from the lab creates an invisible snowman if the color is white. Write a new procedure, (better-snowman height color) that draws a snowman in the given color with every circle outlined in black.

Counting words

As you may recall, we used the following code to count the number of words in a string.

;;; Procedure:
;;;   count-words
;;; Parameters:
;;;   str, a string
;;; Purpose:
;;;   Determine how many words appear in string.
;;; Produces:
;;;   count, a non-egative integer.
;;; Preconditions:
;;;   [No additional]
;;; Postconditions:
;;;   count represents the number of words in the string.
;;;   Note that a "word" is any sequence of characters separated
;;;   by a space.
(define count-words
  (lambda (str)
    (length (string-split str " "))))

1. In your own words, explain how count-words works.
2. Check the behavior of count-words on a few inputs.

Counting letters

Using a lambda, write a procedure, (count-letter str letter), that counts how many times the letter appears in str. For example:

> (count-letter "The vorpal sword went snicker snack" "o")
2
> (count-letter "The vorpal sword went snicker snack" "s")
3
> (count-letter "The vorpal sword went snicker snack" "z")
0

(Hint: Think about the definition of count-words.)

Acknowledgements

The checkerboard example comes from a very old version of CSC 151. We are no longer sure which member or members of the department wrote it.

Bits and parts of this lab come from procedure labs from other versions of CSC 151.