Class 46: Variable-Arity Procedures

Held: Monday, 20 November 2006

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

Related Pages:

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

Notes:

• Time for questions on HW 16.

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.
• Be prepared to reflect.