Class 20: Iteration

Held: Friday, February 26, 2010

Summary: We consider a technique for stepping through the values in a list, and doing some action (e.g., moving a turtle or drawing something with the GIMP tools) for each value in the list.

Related Pages:

• EBoard.
• Lab: Iteration.
• Reading: Iteration.

Notes:

• Reading for Monday: Local Bindings.
• I am eliminating quizzes.
• Assignment 5 is now ready.

Overview:

• A Problem: Drawing Spirals
• A Useful Tool: for-each

The Problem: Making Spirals

• We ended the turtles lab trying to make spirals.
• Basically, you need to move forward a little and turn a little
• Three possible strategies:
  • Move the same amount each time and turn the same amount each time
  • Move a little more each time, and turn the same amount each time
  • Move the same amount each time, and turn a different amount each time
• The first strategy procedures a circle, and is possible to do with repeat.
• The other two strategies seem difficult to do without a lot of hand coding.

The for-each Function

• We'd like to use map, but there are two problems.
  • Officially, you don't know the order in which the function is applied when you do a map; it could be first to last, it could be last to first; it could be all at the same time.
  • We get back a list, which we really don't need.
• Scheme provides for-each for situations like this.
• Like map, but for sequences of actions.
  • for-each is guaranteed to work in order.
  • for-each doesn't return anything.