Held: Tuesday, 24 November 2015

Back to Outline 46 - Analyzing Procedures. On to Outline 48 - Binary Search.

Summary

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

Related Pages

• Reading: Association Lists
• Lab: Association Lists
• EBoard 47 (Source 47)

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

• Continue partners!
• I have confirmed that two senior CS/Studio Art majors will lead a crit of your works on Monday.
  • I'll lead a crit of your code on Tuesday.
• It looks like your projects are going well.
• Exam 4 distributed.

Upcoming Work

• Projects: TONIGHT at 10:30 p.m.
• Reading for Wednesday Search Algorithms
• Today's Lab writeup (due Monday after break)
  • Email to grader-151-01@cs.grinnell.edu
  • Subject: CSC 151 Lab Writeup Class 47 (Your Names)
  • Exercises:
• Exam 4 prologue due Dec. 4.
• Exam 4 due Dec. 7.
• Exam 4 epilogue due Dec. 8.

Extra Credit Opportunities

• If possible, send me these in advance.

Academic

• Any visit to the current show in the Faulconer gallery.
• Future of the Libraries Talk, December 3, Noon, JRC 101.

Peer Support

• Women's Basketball vs. Beloit, Nov. 28 at 3:00 p.m.

Regular Peer Support

• Pals of PALS, pals@grinnell.edu, normally Saturday at 7:45 am (breakfast included; felines only), and Mondays at 4:45. Requires sign up in advance. MORE HELP NEEDED!
• Socrates Cafe', Saturdays, Younker, 2pm. (Not this weekend.)
• Pun Club, Saturdays, 4pm, Way over Younker. (Not this weekend.)

Upcoming Peer Support

• Women's Basketball vs. St. Norbert, Dec. 5 at 3:00 p.m.
• One-act Festival Dec. 5 & 6.
• Choir Concert, Dec. 6 at 2pm in Herrick.
• Women's Basketball vs. Carroll, Dec. 12 at 3:00 p.m.

Miscellaneous

• Choosing a Major, December 1 at 11 a.m.

Other Good Things (No Extra Credit)

• Jingle Bell Holiday December 4.

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