Class 11: Conditionals Lab

Held: Monday, 15 September 2003

Summary: Today we continue our study of conditionals by working with them experimentally.

Related Pages:

• EBoard.
• Reading on Boolean Values.
• Reading on Conditionals.
• Lab on Conditionals.

Assignments

• Read Character Values in Scheme.
• Read String Values in Scheme.

Notes:

• I've posted Notes on homework 1. You can also view your colleague's sites from those notes.
• I forgot to assign the reading on Boolean Values. Sorry. We'll go over some of those issues today.
• Are there questions on Lab writeup 1?

Overview:

• Basic comparisons.
• Predicates.
• Boolean functions.
• The cond operations.
• Lab: Conditionals.

Comparators

• We've noted that if has the form

  (if test truepart falsepart)
  

• What kinds of values can be used for the test?
  • The comparison of two numeric values, e.g., <, >, <=.
  • Comparisons for equality, e.q., equals?, eq?
  • Lots of other stuff, too.

Type Predicates

• One common test takes the form Is this datum a value of this type?
• For example, you can test whether a value is a number?, list?, rational?, and much much more.

Boolean Operators

• Scheme provides a few basic operations for combining Boolean values.
• (not exp)
  • Has the value true when exp has the value false.
  • Has the value false otherwise.
• (and exp1 exp2)
  • Has the value false when either exp1 or exp2 has the value false.
  • Has a not-false value otherwise.
• (or exp1 exp2)
  • Has the value false when both exp1 and exp2 have the value false.
  • Has a not-false value otherwise.
• You'll get to explore the details in more depth in the lab.

The cond Statement

• When you find that you're nesting a whole lot of if statements, you're much better off using the cond statement, which has the form

  (cond (
    (test1 exp1)
    (test2 exp2)
    (test3 exp3)
    ...
    (testn expn)
    (else default)))
  

• The meaning is, in essence, Evaluate each test in turn. When one holds true, use the value of the corresponding expression and stop. If none holds, use default.

Conditionals Lab

• Do the lab.
• Prepare to reflect.