CSC151 2009F, Class 37: Pairs and Pair Structures

Admin:
* Reading for Monday: Trees.  
  * I anticipate updating the reading over the weekend (when I'm not grading
    or raking leaves or helping at the fun run or at Singers or ....).
* You have a writeup on today's lab.
* Quiz 8 (finally) returned.
  * Some are marked "Not counted"
  * Some are maked "Counts"
  * Some are marked "Do you want this to count"
* Are there questions on Assignment 7?
* We'll go over the palette-darken! procedure from the lab on vectors.
* Are there pre-quiz questions on topics from the last week (randomness, vectors, strings, geometric art, ...)?
* When you finish the quiz, start the lab.
* One more week until Rent!

Overview:
* Questions on assignment 7
* Questions on quiz 8
* The palette-darken! procedure
* Pre-quiz questions
* Exploration of pairs
  * Pairs and cons cells.
  * Why care?
* Quiz
* Lab.

About palette-darken!

; Problem: Write palette-darken!
;   "Make it darker"
;   "it is a vector of colors"
;   Apply rgb-darker individually
;   Changing each element in the vector as it goes

; Pretend we only have to change element with index 5
(define palette-darken!
  (lambda (palette)
    (vector-set! palette 5 (rgb-darker (vector-ref palette 5)))))

; Whoops!  Need to do every position

(define palette-darken!
  (lambda (palette)
    ; with each position, pos, from 0 to the length of the palette - 1
    (for-each (lambda (pos)
                (vector-set! palette pos (rgb-darker (vector-ref palette pos))))
         (iota (vector-length palette)))))

; But we don't get to use recursion, and we LOVE recursion.  Really we do.
(define palette-darken!
  (lambda (palette)
    ; Recursively generate the numbers from 0 to (vector-length palette)-1
    (let kernel ((pos 0))
      (cond
        ((= pos (vector-length palette))
         palette)
        (else
         (vector-set! palette pos (rgb-darker (vector-ref palette pos)))
         (kernel (+ 1 pos)))))))

(define palette-darken!
  (lambda (palette)
    ; Recursively generate the numbers from 0 to (vector-length palette)-1
    (let kernel ((pos 0))
      (when (< pos (vector-length palette))
        (vector-set! palette pos (rgb-darker (vector-ref palette pos)))
        (kernel (+ 1 pos))))))

(define rainbow
(define rainbow
  (vector RGB-RED RGB-ORANGE RGB-YELLOW RGB-GREEN RGB-BLUE RGB-INDIGO RGB-VIOLET))


Quiz answers:

Problem 1.
(define delete-char
  (lambda (str pos)
    (string-append (substring str 0 pos)
                   (substring str (+ pos 1) (string-length str)))))

Problem 2.
vector-op! modifies the vector by adding 1 to each element of 
the vector.

Sam's testing of delete-char
> (map (lambda (pos) (delete-char "abcdefghijklmnopqrstuvwxyz" pos)) 
       (iota (string-length "abcdefghijklmnopqrstuvwxyz")))
("bcdefghijklmnopqrstuvwxyz" "acdefghijklmnopqrstuvwxyz"
 "abdefghijklmnopqrstuvwxyz" "abcefghijklmnopqrstuvwxyz"
 "abcdfghijklmnopqrstuvwxyz" "abcdeghijklmnopqrstuvwxyz"
 "abcdefhijklmnopqrstuvwxyz" "abcdefgijklmnopqrstuvwxyz"
 "abcdefghjklmnopqrstuvwxyz" "abcdefghiklmnopqrstuvwxyz"
 "abcdefghijlmnopqrstuvwxyz" "abcdefghijkmnopqrstuvwxyz"
 "abcdefghijklnopqrstuvwxyz" "abcdefghijklmopqrstuvwxyz"
 "abcdefghijklmnpqrstuvwxyz" "abcdefghijklmnoqrstuvwxyz"
 "abcdefghijklmnoprstuvwxyz" "abcdefghijklmnopqstuvwxyz"
 "abcdefghijklmnopqrtuvwxyz" "abcdefghijklmnopqrsuvwxyz"
 "abcdefghijklmnopqrstvwxyz" "abcdefghijklmnopqrstuwxyz"
 "abcdefghijklmnopqrstuvxyz" "abcdefghijklmnopqrstuvwyz"
 "abcdefghijklmnopqrstuvwxz" "abcdefghijklmnopqrstuvwxy")