Class 35: Variable-arity procedures

Held Tuesday, October 31, 2000

Summary

Today we visit variable-arity procedures, procedures that can take differing numbers of arguments.

Notes

• Today's stuff:
  • Reading: Variable-arity procedures
  • Lab: Variable-arity procedures.
• Are there questions on exam 2?
• In case you hadn't noticed, I've started marking the preconditions for each procedure and I expect you to do the same.
  • If the procedure explicitly checks the precondition, mark it as [Verified] or [Checked].
  • If the procedure does not explicitly check the precondition, mark it as [Unverified] or [Unchecked]
• I've also tried to make it clear that there are many different things you might say about postconditions (and perhaps you should make it a\ point to document default postconditions).
• Make it a practice to document your procedures with the six Ps
  • Procedure name
  • Paramters
  • Purpose
  • Produces (return value)
  • Preconditions
  • Postconditions
• I've noticed that many of you print the labs and readings. Next semester (when I've actually managed to put all the stuff together), should I simply hand out printed copies to conserve paper (I'll print two-up and double sided)?
• Happy Halloween!

Overview

• Definition: arity, the number of parameters a procedure expects.
• Some examples:
  • cons
  • car
  • expt
  • list
  • quote
  • map
• As you may have noted, some procedures do not have a fixed number of parameters. We call such procedures variable-arity procedures.
• Can you write your own? Of course.
• Two basic forms.
• First form: 0-or-more parameters:

  (lambda args
    body)
  

• args is a list.
• Example

  > (define first 
      (lambda stuff (car stuff)))
  > (first 1 2 3)
  1
  > (first 3 4 5 6 7)
  3
  > (first 3)
  3
  > (first)
  car: expects argument of type <pair<; given ()
  

• Observation: Sometimes you want to require parameters.
• Second form: 1-or-more parameters

  (lambda (arg1 . args)
    body)
  

• arg1 is a single value; args is a list.
• Example:

  *gt; (define first
    (lambda (arg . args)
       arg))
  > (first 1 2 3)
  1
  > (first 3 4 5 6 7)
  3
  > (first 3)
  3
  > (first)
  procedure first: expects at least 1 argument, given 0
  

• Variations: 2-or-more-parameters

  (lambda (arg1 arg2 . args)
    body)