Class 04: Procedures

Held: Monday, 26 January 2004

Summary: Today we consider how you can write your own procedures in Scheme.

Related Pages:

• EBoard.
• Outline of Class 4.
• Reading on Procedures in Scheme.
• Lab on Procedures in Scheme.

Assignments:

• Read Conditional evaluation in Scheme.
• Read Boolean values in Scheme.

Notes:

• Are there questions on homework 1? Note that you may not work alone on this homework!

Overview:

• Why define your own procedures?
• How to define your own procedures.
• Lab on user-defined procedures.

User Defined Procedures

• It's clear that programmers often want to (and need to) define their own procedures.
• User-defined procedures can add clarity to a program.
  • Rather than looking at how code does something, the user of a procedure can focus on what the code does.
• Programmers can avoid repetitive (and, therefore, error prone) code.
  • Rather than retyping the same code again and again, just changing a few values, a programmer can give a name to the same code.
• How do you define your own procedures? Using the following template:

  (define your-procedure
    (lambda (param1 ... paramn)
      expression-possibly-involving-parameters))
  

• For example,

  (define square
    (lambda (val)
      (* val val)))
  

• There's another way to define procedures in Scheme, but we prefer that you avoid it. So that you can read code others write, we'll look at it quickly.

  (define (your-procedure param1 ... paramn)
     expression-possibly-involving-parameters)
  

• For example,

  (define (square val)
    (* val val))
  

• You can (and should) document your procedures so that others can understand what they are supposed to do.
  • We'll return to documentation style later this semester.

Lab on User-Defined Procedures

• Do the lab.
• Be prepared to reflect (e.g., to describe the most important or most confusing thing you dealt with today).