CSC153 2004S, Class 15: Vectors

Admin:
* IF YOU MISS CLASS, PLEASE DROP ME A NOTE EXPLAINING WHY (preferably in advance)
* Why is Friday the 13th unlucky?
  * 13 is unlucky because certain religions are concerned about the 13th apostle (Judas)
  * Friday is unlucky because those same religions note a number of events happened on Friday (It's on the Internet, so it must be true.)
  * Theorem: It is no less common than any other date.
  * To maximize unluckiness.
* Is "Internet" generally capitalized?
  * "Internet" should be capitalized, because it's a proper noun.
* Is "anal retentive" hyphenated?
  * It depends on how you use it.
  * "Sam is anal retentive." is not hyphenated
  * "Sam is an anal-retentive editor." is hyphenated.
* Questions on the exam?
  * Sam unavailable this weekend.
  * Ask questions on Monday.
* Questions on yesterday's class?
* Cool new reading: Higher-order procedures

Overview:
* Strengths and weaknesses of lists
* An alternative: Vectors
* Key vector procedures
* Lab

Do you already know all of this from your work with arrays and linked lists in whatever languages you know?

Good things about lists (in Scheme):
* Defined recursively: Either null or cons of a value and a list.
* Dynamic: Easy to extend (as much as you want) (or shrink).
* Heterogeneous: Can contain multiple types of data

Lists have some limitations:
* Lists are not mutable (or at least Sam hasn't taught you how to mutate them)
* Accessing elements in lists is slow. ("Random" access is not random.)
* Heterogeneous: Can contain multiple types of data

Vectors are the solution!
* Mutable
* Fast to access elements
* Not defined recursively
* Not dynamic

How do we recurse over vectors?
* With a list, we can cdr
* With vectors, we can use an accompanying counter that we iterate recursively

Try the lab!

Observation: 
* vector-set! does not return the vector
* You can write your own variant that does

* The name of a mutator should always end with !
* On the exam, 
> (value->string (list 1 2 3))
"(1 2 3)"
* When you need to do more than one thing in a branch of an if , use begin
(if (test)
    (begin
      (vector-set! vec ....)
      (recurse vec (+ pos 1)))
    ...)