Class 09: Procedure Definitions

Held: Thursday, 11 September 2003

Summary: Today we begin to consider how you can write your own procedures.

Assignments

• Read Conditional Evaluation in Scheme.
• Write up The lab on procedures. Please follow the guidelines on laboratory writeups.

Notes:

• Let's start class with a moment of silence for those who lost their lives two years ago and those who have lost their lives in the followup actions.
• I'll return homework 1 on Monday.

Overview:

• Why define your own procedures?
• How to define your own procedures.
• Lab.

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)))
  

• You can (and should) document your procedures so that others can understand what they are supposed to do.

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).