Class 39: Variable-Arity Procedures

Held Thursday, April 12, 2001

Summary

Today we consider how and why to write procedures that can take different numbers of arguments in different situations.

Notes

• For tomorrow, read Procedures that Return Multiple Values.
• Monday, April 16 at 4:15 p.m. in Noyce Science Center 2400, join artist Irve Dell, the Department of Mathematics and Computer Science, and the Faulconer Gallery to celebrate the installation of permanent art works in the Math/CS stairwell.
• Only today and tomorrow for questions on exam 2
  • Question 1: Grading
    • What should you do about fractional grades? Round them.
    • How do you write to a file? Here's some sample code.

      (let ((outport (open-output-file "example")))
        (display "Hello There" outport)
        (newline outport)
        (display "Okay, now I'm done" outport)
        (newline outport)
        (close-output-port outport))
      

  • Question 3: Currying
    • I'd prefer if you wrote cmember? directly, rather than implementing it with a call to member.
    • The name of cmember? suggests that it returns only #t or #f.

Overview

• Definition of arity
• Why have variable-arity procedures
• How to write variable-arity procedures
• Lab

Variable-Arity Procedures

• Def. Arity: The number of parameters a procedure expects.
• Some procedures naturally take varying numbers of parameters:
  • + takes 0 or more parameters.
  • display takes 1 or 2 parameters.
  • map takes 2 or more parameters.
• It would be nice to be able to write our own procedures of variable arity (that is, varying numbers of parameters).
• Zero or more parameters:

  (lambda params
    body)
  

  • params is not parenthesized.
  • In your body, treat params as a list.
• One or more parameters:

  (lambda (first . remaining)
    body)
    + In your body, treating remaining as a list.
  

• Two or more parameters:

  (lambda (first second . remaining)
    body)
    + In your body, treating remaining as a list.
  

Lab

• Do the laboratory entitled Variable-Arity Procedures.