CSC151 2010S, Class 17: Exam 1 Discussion

Overview:
* General issues.
* What is functional programming?
* Issues with particular problems.
* Additional Q&A
* Lab (if time).

Admin:
* Reading for tomorrow: Turtle Graphics.
* EC for Wednesday's panel on Open Information (4:30 pm Burling).
* EC for Thursday's convo or Wednesday night's talk (where science and religion overlap)
* EC for Thursday's CS Extra (4:15 3821).
* EC for Friday's CS Table (noon, JRC PDR).
* Reminder: Sleep/Rest, Eat well, Wash hands
* Are there questions on Assignment 4?

General exam comments
* Test taking strategies
  * Look at the whole exam first
  * Don't take the exam all at once
  * Ask Sam for help!
  * Start early

What is functional programming?
* Computation through the evaluation of 
  expressions
  * Amazingly, this happens in almost every
    programming language
* Computation that avoids mutable data
  * mutable == changing/changeable
  * Our drawings are an instance of immutable
    data
    * Can be counter-intuitive
  * Numbers are an instance of immutable data
  * Once we've named a value, the name retains
    the same value
  * But we've seen mutable data: images are 
    mutable
    * The same operation will behave differently
      at different states of the program
      (image-get-pixel image 2 3)
* Procedures are functions in the mathematical
  sense
  * It provides an output for an input
  * It does math!
  * "The mathematical sense" => Given the same
    inputs, a function always provides the
    same output
* Functional languages generally don't give you access to "state"
  * "state" is something that is changeable
  * In Scheme, we do sometimes change the state with the ! procedures
  * The state of an image is the setting of each pixel of the image
  * The state of a named value is the value associated with the name
  * You can change part of a state (e.g., one pixel)
  * The state of the computer is the setting of every memory location (plus a bit more)
* Scheme is NOT a pure functional language
* Citations of Web pages
  * Author
  * Title
  * Date of access
  * ...

(define simple-circle
  (drawing-scale
   (drawing-shift drawing-unit-circle 0.5 0.5)
   50))

(define drawing-render
  (lambda (drawing)
    (image-show
     (drawing->image drawing 
                     (inexact->exact (round (drawing-right drawing)))
                     (inexact->exact (round (drawing-bottom drawing)))))))
  

(define drawing-hpair
  (lambda (drawing1 drawing2)
    (drawing-group
     drawing1
     (drawing-hshift drawing2 (drawing-right drawing1)))))

(define drawing-vpair
  (lambda (drawing1 drawing2)
    (drawing-group
     drawing1
     (drawing-vshift drawing2 (drawing-bottom drawing1)))))

(define warhol
  (lambda (drawing)
    (drawing-vpair
     (drawing-hpair 
      (drawing-recolor drawing "red")
      (drawing-recolor drawing "green"))
     (drawing-hpair 
      (drawing-recolor drawing "blue")
      (drawing-recolor drawing "yellow")))))

(define drawing-background
  (lambda (drawing color border)
    (drawing-group
     (drawing-vscale
      (drawing-hscale 
       (drawing-recolor
        (drawing-shift drawing-unit-square 0.5 0.5)
       color)
       (+ border border (drawing-width drawing)))
      (+ border border (drawing-height drawing)))
     (drawing-shift drawing border border))))

(define andy
  (lambda (drawing)
    (drawing-vpair
     (drawing-hpair
      (drawing-background drawing "red" 10)
      (drawing-background drawing "blue" 10))
     (drawing-hpair
      (drawing-background drawing "green" 10)
      (drawing-background drawing "yellow" 10)))))