Class 20: Association Lists

Held Wednesday, October 2, 2002

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.

Useful Files

• The reading on association lists
• The lab on association lists

Assignments

• Exam 1 distributed.
• Read Vectors.

Due

• Homework 4: Recursion lab writeup.

Notes

• My bad week continues. I managed to leave my car windows open last night. Bad plan Sam.
• I have not yet graded homework 3. I will probably grade homework 4 first, since it is more relevant to the exam.

Overview

• Databases and dictionaries
• Lists as dictionaries
• 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 (or tables within 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.

Lab

• Do the lab.
• You won't finish today. Try to work on it tonight and tomorrow. Come with questions on Friday.