Class 12: Variable-Arity Procedures

Held: Friday, 7 February 2003

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

Related Pages:

• EBoard.
• Reading on Variable-Arity Procedures.
• Lab on Variable-Arity Procedures.

Assignments:

• Read Characters in Scheme.
• Read Strings in Scheme.

Notes:

• Sorry about missing Wednesday's class. I was home with a sick child.
• Are there any questions on Laboratory Writeup 2?
• Expect exam 1 (take-home) a week from today.

Overview:

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

Some Terminology

• Definition: Parameter: A value passed to a procedure.
  • Formal parameter: A parameter as it appears in the procedure definition.
  • Actual parameter: A parameter as it appears in the procedure call.
• Def: Arity: The number of actual parameters a procedure expects.
• Common variants:
  • A unary procedure takes one actual parameter.
  • A binary procedure takes two actual parameters.
  • A variable-arity procedure can take varying numbers of actual parameters.

Variable-Arity Procedures

• 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, treat remaining as a list.
  

• Two or more parameters:

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

• Three or more parameters:
  • I bet you can figure it out.

Lab

• Do the laboratory entitled Variable-Arity Procedures.
• Be prepared to reflect.