# Class 21: Boolean Values and Predicate Procedures

Back to Naming Local Values. On to Conditionals.

This outline is also available in PDF.

Held: Friday, 2 October 2009

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:

• Don't forget the cool programming languages talk today at noon.
• Exams not graded. Barring continued unexpected circumstances, they will be returned Monday.
• Are there questions on assignment 5?
• Are there questions on problem 6 on Wednesday's lab?
• It's time for another Lab writeup. Do problems 2-5.
• Have a great Family Weekend!

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

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

Copyright © 2007-9 Janet Davis, Matthew Kluber, Samuel A. Rebelsky, and Jerod Weinman. (Selected materials copyright by John David Stone and Henry Walker and used by permission.) This material is based upon work partially supported by the National Science Foundation under Grant No. CCLI-0633090. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. This work is licensed under a Creative Commons Attribution-NonCommercial 2.5 License. To view a copy of this license, visit `http://creativecommons.org/licenses/by-nc/2.5/` or send a letter to Creative Commons, 543 Howard Street, 5th Floor, San Francisco, California, 94105, USA.