CSC151.02 2003F, Class 42: Association Lists

Admin:
* Homework: Tail recursion lab (including extra)
* No reading this weekend
* Extra credit for attending the Roots of Renewal concert (1/2 point)

Overview:
* Databases
* Scheme's Association Lists
* The assoc procedure
* Useful variants
* Lab

Question: Who are the richest Americans?
* Larry Ellison - Databases (Oracle)
* Bill Gates - Dropping out of Harvard; Stealing/buying good software ideas
* Warren Buffett - Buying good companies

When we talk about computing, we should probably talk about Databases.

What is a database?
* An organized, possibly indexed, collection of information that eases analysis and retrieval
* At the root of most databases is a simple idea: You can index information by a "key"
  * The key is the thing used to retrieve the information; it's the entry in the index

The simplest form of database is the "dictionary" (also called a "keyed table")
* A collection of keyed values.

In Scheme, we can implement a dictionary as a list of lists.
* The key of each member list is the car of that list
* We call this structure "an association list"
* Rather than writing your own "search" procedure, you can use assoc

If Scheme did not provide assoc, we could implement it!

We could therefore build interesting variants
* Find all matching elements
* Use a predicate rather than a key as the first parameter
* Both [Hmm ... that's select]

Do the lab!