Class 16: More Recursion with Lists

Held Wednesday, September 25, 2002

Summary

Today we continue our exploration of recursion over lists through experimentation and reflection.

Due

• Homework 3.

Notes:

• Many of you sent me notes at 3 a.m. indicating that you weren't able to get the homework done because of unknown errors.
  • I will not penalize you for the lateness on this assignment.
  • This offer is only available to those who show up for class.
  • I'll wander around during lab to see if I can fix your problems.
• For the future: I'd rather that you go to sleep before midnight than that you get my assignments in on time.
  • And as today shows, those who need to work beyond midnight don't often finish their assignments anyway.
• To continue our "P" theme, I brought pudding and pastries as well as Pop-Tarts (and today I brought real Pop Tarts as opposed to other kinds of toaster pastries).
• The focus of today's class is the recursion lab.
• No, I do not yet have homework 2 graded.
• If you haven't done so already, please read Recursion with Natural Numbers.

Overview

• Recursively-defined types
• Reflection
• Lab: Repetition with Recursion
• More Reflection

Recursively Defined Data

• You may not have gotten this idea from the readings, but we define more than procedures recursively: We also define some data recursively.
• Lists: A list is either
  • the empty list (base case)
  • the cons of a value and a list (recursive case)
• Natural numbers: A natural number is either
  • the value 0 (base case)
  • one plus a natural number
• ...
• As you may have noticed, you can use these recursive definitions to ground your recursive procedures.

Some Reflection

• As I look at my outline from the last time I taught this class, I see that many people in the class had problems with the square each element in the list problem. You folks seem to have done much better. Congrats!
• Are there other problems you've encountered or observations you've made in the initial work on the lab?
• Did any of you glean interesting ideas from the second reading on recursion?

Lab, Continued

• Continue your work on the first recursion lab.