Class 18: Pairs

Held Wednesday, February 21, 2001

Summary

Today we visit some of the behind the scenes structures in Scheme, particularly the pairs that cons creates.

Assignments

• Read Association Lists.

Notes

• Reminder: You should either do homework 3 or laboratory writeup 1.
  • You should, however, look over both.
  • I'm pretty sure that the first exam will include some ideas from the homework and the writeup.
  • Are there questions on either?
• Reminder: I will distribute exam 1 on Friday and it will be due the following Friday.
  • If you'd like, you can look over the first exam from last semester and my notes on that exam.
  • This year's exam will probably be somewhat shorter, but you should still be ready to spend up to ten hours on it.
• Don't forget the cool talk on Thursday at 4:30, I can change your Web page and you can't do anything about it (known more formally as Web Raveler: An Infrastructure for Transforming Web Pages.
• There still seem to be some serious questions about how CGI works, so we'll spend a few more minutes going over the philosophy and the practice.

Overview

• Behind the scences in Scheme: Memory
• Pairs and Cons cells
• Dotted pairs
• Lab

Behind the Scenes

• In Scheme, every value that you create takes up computer memory.
• Typically, the memory allocated to a value includes the type of the value and the contents of the value.
• As you might guess, different values have their contents arranged differently.
• Scheme also keeps a table (or tables) that associates names with values in memory. (Picture to be drawn in class.)
• Why is this important? Because it can help us think about some things.

Cons Cells

• Perhaps the most interesting values are the things that cons creates. We often call these cons cells or pairs
• The first value of a pair is a reference to the first parameter to cons. The second value is a reference to the second parameter.
• Let's look at some examples.

Dotted Pairs

• Although we've used lists as the second parts of each cons cell, you can use other values. In this case, you get what is called a dotted pair.
• The shorthand for a dotted pair is (x . y).
• You'll also see the dot at the end of a long sequence of cons cells.

Lab

• Any questions?
• Do the work.