Class 31: Vectors

Held: Monday, 27 October 2003

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.
• Reading on Vectors.
• Lab on Vectors.

Assignments

• Read Output in Scheme.
• Read Input in Scheme.

Notes:

• I hope you had a great vacation.
• Expect exam 2 tomorrow.

Overview:

• Problems with lists.
• A solution: Vectors.
• Side note: The begin construct.
• 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.

Detour: 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 vector-set! or display, that have side effects.
  • Otherwise, all you're doing is computing values and then forgetting about them.

Lab

• Do the lab.
• Note that I'll probably ask you to reflect in the next class session.