Class 32: Vectors

Held: Friday, March 16, 2007

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.

Related Pages:

• EBoard.
• Lab: Vectors.
• Reading: Vectors.

Due

• Project Report 1

Notes:

• Thanks for your patience on Wednesday. Eldest son came in first, so we're heading off to state in a month or so. (On a Saturday, this time.)
• Have a great break!

Overview:

• Problems with lists.
• A solution: Vectors.
• Lab.

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?).

An Alternative: Vectors

• Vectors provide an alternative to lists.
• They have two primary advantages:
  • Vectors are indexed: You can quickly access elements by number.
  • Vectors are mutable: You can change the elements of a vector.
• In order to obtain these benefits, vectors lack some key features of lists. In particular,
  • Vectors are static: Once you've created a vector, you cannot change its length.
• Some key vector procedures:
  • (vector val1 ... valn): Create a vector
  • (make-vector length val): Make a vector of specified length, with duplicates of val as the contents.
  • (vector-ref vector position): Extract a value from a vector.
  • (vector-set! vector position newvalue): Change an element of a vector.

Lab

• Do the lab.