Held: Monday, 21 April 2014

Back to Outline 44 - Pause for Breath. On to Outline 46 - Binary Search.

Summary

We consider association lists, a simple, but useful, technique for organizing tables of information.

Related Pages

Overview

• Storing information in tables.
• Representing table entries as lists.
• Representing tables as lists.
• Association lists: Scheme's standard table representation.
• Implementing key association list procedures.

Administrivia

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.
• A database is a mechanism for storing data so that you can easily access the data you need.
• One simple database activity is looking up values by keys.
• Databases that provide only that activity are called dictionaries.

Association Lists

• In Scheme, dictionaries are typically implemented with a data structure known as the association list.
• An association list is 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.

Variants of Association Lists

• Given that we can write our own assoc procedure, we can easily implement a number of interesting variants of association lists.
• For example, if the same key appears multiple times in the association list, we might return all matching values (rather than the first matching value).
• Similarly, instead of searching by key, we might search by predicate.

Lab

• Do the lab