CSC151 2010S, Class 24: Revisiting Lists

Overview:
* Lessons from Wednesday
* Lists, As We Know Them
* Lists, Some New Perspectives
* Lab

Admin:
* Reading for Monday: Recursion Basics 
  * Warning: the reading is complex
  * Expect to bring questions
* Are there questions on Assignment 6?
* Skip problems 5 & 6 on today's lab.
* It appears that we'll have lots of missing students today.
* EC for today's Peace Studies sessions (1p, 3pm, or 4:45 pm).
* Exam 2 to be distributed Tuesday.

Reflection and Questions from Wednesday's Lab

(define rgb-brighter?
  (lambda (rgb1 rgb2)
    (if (> (rgb-brightness rgb1)
           (rgb-brightness rgb2))
        #t
        #f)))

vs.

(define rgb-brighter?
  (lambda (rgb1 rgb2)
    (> (rgb-brightness rgb1)
       (rgb-brightness rgb2))))

---
(define rgb-brighter-of-two
  (lambda (rgb1 rgb2)
    (if (rgb-brighter? rgb1 rgb2)
        rgb1
        rgb2)))

---
(define rgb-4grey
  (lambda (rgb)
    (if (> 25 (rgb-brightness rgb))
        grey0
        (if (and (> 50 (rgb-brightness rgb))
                 (< 25 (rgb-brightness rgb)))
            grey1
            (if (and (> 75 (rgb-brightness rgb))
                     (< 50 (rgb-brightness rgb)))
                grey2
                (if (and (> 100 (rgb-brightness rgb))
                         (< 75 (rgb-brightness rgb)))
                    grey3))))))

vs

(define rgb-4grey
  (lambda (rgb)
    (if (> 25 (rgb-brightness rgb))
        grey0
        (if (> 50 (rgb-brightness rgb))
            grey1
            (if (> 75 (rgb-brightness rgb))
                grey2
                grey3)))))

Write a new predicate, (image-contains-selection?  img left top  width height), that determines whether a selection using left, top, width, and height is likely to be within img. 
* Explanation (image-select-ellipse! img REPLACE 
left top width height)
  * left must be less than the width of the image
  * top must be less than the height of the image
  * (Note: Left can be negative.)
  * left can't be more negative than the width of the selection
    * (< 0 (+ left width))
  * Do the same thing with top
    * (< 0 (+ top height))
(define image-contains-selection?
  (lambda (img left top width height)
    (and (< left (image-width img))
         (< top (image-height img))
         (< 0 (+ left width))
         (< 0 (+ top height))
         (< 0 width)
         (< 0 height))))
MAY NOT BE EXACT
      
Lists, As We Know Them

Lists, Some New Perspectives

Lab