Held: Friday, 4 October 2013

Back to Outline 20 - Naming Local Values. On to Outline 22 - Conditionals.

Summary

We consider the ways in which Scheme supports yes/no questions and procedures that can be used to ask such questions. We also see how such procedures can provide another form of control.

Related Pages

Overview

• Boolean values.
• Predicates - Procedures that return Boolean values.
• Combining booleans with and and or.
• Lab.

Administrivia

Preparation

• What did you learn from the reading? (Let's try to summarize for each other.)
• What questions do you have?

or and and as control structures

• How does normal procedure application work?
  • Evaluate all the parameters
  • "Plug them in to the body"
  • Evaluate the body
• How does or work?
  • It evaluates each parameter, in turn, until one holds (or it runs out of parameters).
  • As soon as one holds, it stops and returns that value.
• How does and work?
  • It evaluates each parameter, in turn, until one fails to hold (or it runs out of parameters).
  • As soon as one fails to hold, it stops and returns #f.
• This evaluation strategy means that we can use and and or to evaluate a sequence of expressions, stopping when we get the first acceptable one.

Lab

• Work on the lab.
• Be prepared to discuss!