# Class 22: Boolean Values and Predicate Procedures

Back to Naming Local Values. On to Conditionals.

This outline is also available in PDF.

Held: Tuesday, March 2, 2010

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:

Notes:

• Reading for Wednesday: Conditionals.
• Advance warning: Exam 2 distributed next Tuesday; substitute here W-F.
• There was a racial bias incident over the weekend. We will discuss it in class if folks find it appropriate.
• EC for Thursday's Thursday Extra: (not sure).
• EC for Friday's CS Table: Notes from an alum.
• Questions and tips on Assignment 5.

Overview:

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

## 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!

Back to Naming Local Values. On to Conditionals.

Disclaimer: I usually create these pages on the fly, which means that I rarely proofread them and they may contain bad grammar and incorrect details. It also means that I tend to update them regularly (see the history for more details). Feel free to contact me with any suggestions for changes.

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Samuel A. Rebelsky, rebelsky@grinnell.edu