CSC151 2007S, Class 10: Representing Images as Lists of Spots

Admin:
* Assignment 3 due!
  * PLEASE USE THE CORRECT TITLE!
* Reminder: Quiz 2 on Friday (some code reading, some code writing).
* Reminder: Exam 1 distributed on Friday.
* Reminder: Purcell convocation tomorrow.
* Reading for Friday: Documenting Procedures.
* Some stuff from yesterday
* So, is Grinnell too classist in its reaction to the snow?

Overview:
* Stuff from Yesterday
* Lists in Scheme.
* Basic list operations.
* Spot lists: Another perspective on images.

==Stuff from Yesterday==

* So, how do we write grade-count?

;;; Procedure
;;;   grade-count
;;; Purpose
;;;   To decide what extra "count" to give for a grade, using
;;;   the policy "80 and above counts as 1"; anything else counts
;;;   as 0.
;;; Produces:
;;;   count, an integer (either 0 or 1)

;;; Strategy:
;;;   divide the raw score by 100
;;;   0..79 => .0.. .79
;;;  80..100 => .80 .. 1.00
;;;   Add .2
;;;   0..79 => .2.. .99
;;;  80..100 => 1.00 .. 1.20
;;; Truncate!
;;;   0..79 => 0 .. 0
;;;  80..100 => 1 .. 1

;;; Alternate strategy: Divide by 80 and use the floor

;;; ALternate strategy from other 151
;;; (modulo (bound grade 79 80) 79)
(define bound
  (lambda (val lower upper)
    (min (max val lower) upper)))

(define grade-count1
  (lambda (grade)
    (truncate (+ 0.2 (/ grade 100)))))

(define grade-count2
  (lambda (grade)
    (truncate (/ grade 80))))

(define grade-count3
  (lambda (grade)
    (modulo (bound grade 79 80) 79)))


Which is better?
* divide by 100, add .20, truncate because it "feels better"
* bound and mod, because it's more straightfoward and has fewer operations
* grade-count2 is easier to understand/it's more intuitive
* grade-count3 might be easier to change
* grade-count2 and grade-count1 would be easier for other values
* grade-count3 requires the prior definition of "bound"

===Other ways to look at modulo===

Drawings on the real white board.
TONS OF EXTRA CREDIT IF YOU WRITE A PROGRAM TO REPLICATE THOSE
DRAWINGS I JUST MADE

==Lists in Scheme==

* Innovation: Lists as a built-in type
* Wonderful idea: Easy to build collections from lists

==Basic list operations==

* BUild lists like you build drawings
* Start with a simple list: The empty list
* Create new lists by adding to that list
  * ONLY AT THE FRONT

==Spot lists: Another perspective on images==