Class 16: Association lists

Held Wednesday, September 20, 2000

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.

Notes

• Lab: Association lists
• Reading for Friday: Vectors
  • The reading won't be ready until this afternoon.
• A reminder: There is almost always a link to the current lab at the top of the current day's outline.
• Another reminder: You can certaionly come to see me when you need help
  • I haven't had many visitors, so I'm not sure if you're all getting the material well or ...
  • I'll be in the office until about 2:45 today (I'm picking up my son after school) and most of the day tomorrow.
• As I hinted at in the reading on association lists, there will be a quiz today.

Overview

• Databases
• Association lists
• Sequential search

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 (key,value) pairs.
• 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 pair in the list is the key, 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.