CSC151 2007S, Class 32: Vectors

Admin:
* Due: Project Report 1
  * Code + Text Files + Report
* Assigned: Exam 2
* 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.

/What is a List?/

* A potentially empty collection of values
* Primary operations:
  * car - the first element in the list
  * cdr - the rest of the list
  * cons - add something to the front
  * null - build an empty list
  * null? - check if a list is empty
* Secondary operations:
  * append
  * list-ref gives you "any elment you want"

* Suppose we have the list, letters = ("a" "b" "c" "d" "e")
* How do you get element 3?
  (list-ref letters 3)
  (car (cdr (cdr (cdr letters))))
  * Whichever you use still implicitly does three cdrs
* Suppose you want to set element 2 to "k"
  * You'll need to rebuild the list
    (define revised-letters 
      (cons (car letters) 
            (cons (car (cdr letters))
	          (cons "k"
		        (cdr (cdr (cdr lst)))))))

These are the two big flaws of lists:
* Slow to access elements later in the list
* Lists are not (naturally) mutable 

The solution: Vectors
* Mutable
* Equal access time for each element

For 1c: You'll need both make-vector and vector-set!

(define vec (make-vector 2))
(vector-set! vec 0 ...)
(vector-set! vec 1 ...)
vec

NOTE: vector-set! does not return a value.  Hence, you only use it with
vectors that you have already named!