Class 19: Association Lists

Held Thursday, February 22, 2001

Summary

Today we visit a database-like application of Scheme: Association lists. Association lists allow you to associate information with a key and then look up the value by the key.

Assignments

• Read Preconditions and Postconditions.

Notes

• A few of the CS majors and I were discussing you folks, particularly the problems you're having with CGI. They wanted to tell me that it seems to them that you know much more than they did at this point in their careers. So ... congratulations! and don't be too stressed about it.
• Are there any final questions on homework 3 or lab writeup 1?
  • Yes, you are allowed to rely on (and write) helper procedures when you write your own procedures.
• Don't forget today's really cool CS talk: I can change your Web page and you can't do anything about it (known more formally as Web Raveler: Architectures for Transforming Web pages).

Overview

• Databases
• Lists as databases
• assoc, a procedure for searching lists
• Lab

Simple Database Problems

• Databases are among the most common applications of computers.
  • After all, there's a reason that Larry Ellison is nearly as rich as Bill Gates.
• At the simplest level, a database is a mechanism for storing data so that you can easily access the data you need.
• Simple databases in which you can associate a value (or values) with a particular key are called dictionaries.

Association Lists

• In Scheme, dictionaries are typically implemented with a data structure known as the association list.
• An association list is simply a list of elements each of which has a key as its car.
• You can use the (assoc key list) procedure to look up values by key.

Searching in Association Lists

• Suppose Scheme didn't include assoc. How would you write it? Probably recursively.
• If the list is empty, it does not contain the value.
• If the key of the first element in the list is the key we're looking for, return the corresponding value.
• Otherwise, look in the rest of the list.
• This technique is called sequential search.
• We'll turn that into Scheme together.

Lab

• Do the lab.
• You won't finish today. Try to work on it tonight.