Class 21: Vectors

Held Friday, October 4, 2002

Summary

Today we consider vectors, which serve as alternates to lists. Vectors differ from lists in three primary ways: vectors have fixed size, vectors are indexed, and vectors are mutable.

Today's Work

• The reading on vectors
• The lab on vectors

Assignments

• Read Naming Values with Local Bindings.

Notes

• I've graded homework 4.
  • I've emailed each of you some notes on your assignment.
  • You should read my notes on homework 4 before you turn in the exam.
  • We'll look at some issues with all-in-range.
  • We may also look at some other procedures.
• I do not plan to grade homework 3.
  • I don't think you'd like me to grade it.
  • When time permits, I will release some notes on it.
• Are there questions on exam 1?
  • No, you don't need to use assoc in your implementation of swap-name-and-phone.
• The Putnam Mathematics Exam (a full day of challenging mathematical problems) is on Saturday, 7 December, from 9am to 5pm.
  • Email gumben@grinnell.edu to sign up.
  • You're guaranteed to do at least as well as I did when I took it twenty years ago.

Overview

• Problems with lists
• The begin construct

List Deficiencies

• Now that we've worked with lists for a while, we've identified some things that make lists inappropriate for some situations.
  • List are expensive to use; to find the length of a list or to access an element late in the list, you need to cdr down the list.
  • Lists are fixed; you can't easily change an element of a list.
• At the same time, there are some advantages to lists:
  • Lists are dynamic; it is easy to grow and shrink a list.
  • Lists are inherently recursive; the type is defined recursively.
  • Lists are simple; you can build almost every list operation through just a few basic operations (car, cdr, null, and null?).

Sequencing Statements with begin

• We sometimes find that we need to do more than one thing when working with side-effecting operations.
  • For example, we might modify a vector and then recurse, as in list->vector from the reading.
• Scheme provides the simple begin construct for do more than one thing in sequence.
• Here's the form

  (begin
    expression-1
    expression-2
    ...
    expression-n)
  

• Note that this form is only useful when you have operations, like set!, that have side effects.
  • Otherwise, all you're doing is computing values and then forgetting about them.