TeachScheme! 2000

- demo ping-pong.  

  Ping-pong requires:
  -- dealing with conditional situations
  -- dealing with compound data
  -- dealing with graphics (we don't teach this though, since it's
     not portable learning -- use lib instead)

- claims and conditionals

  Claims are true or false: 
    given any x on numberline, only one of 
    -- x = 0
    -- x < 0
    -- x > 0 
    holds; examples!

  Scheme syntax: [show contracts!]
    -- (= x 0) 
    -- (< x 0)
    -- (> x 0)

  Computation: Booleans! [note: not 0 or 1.  booleans are a distinct
                          kind of value]

  Consider interval on number line: 
    all numbers between 0 and 5. 
  To describe it, we need compound claims: 
    (and (< 0 x) (< x 5))

  Computation: reduce one to true, than the next one, etc. 
    [hand eval on a couple of examples: x = 3, x=-2]

  If have boolean values, should be able to create functions that
  return booleans.

  Boolean-value functions: 
    -- Is the ball inside some box?         if not, game's over
    -- Is the ball inside of some interval? if yes, it hit paddle

  Example: ;in_interval : num num num -> boolean
           ;true if n between lo and hi, inclusive
           (define (in-interval n low hi) ...)

  Conditional functions: 

    Sometimes the method for computing results differs depending
    on what the input is. We need CONDITIONAL EXPRESSIONS: shape
    (question-answer); use; evaluation.

  Example: 
    -- in which interval did the ball land? 
    -- how much is your pay-back rate? 
    -- ;sign : num -> symbol
       ;returns 'positive, 'negative, or 'zero based on sign of input
       (define (sign n) ...)
    -- ;pepper-scale : num[>=5000] -> symbol
       ;returns pepper name based on scoville-index
       (define (pepper-scale n)
         (cond [(and (>= n 5000) (< n 30000)) 'serrano]
               [(and (>= n 30000) (< n 50000)) 'cayenne]
               [(and (>= n 50000) (< n 100000)) 'thai]
               [(>= n 100000) 'habenero]))
       [note: the scoville-organoleptic test measures the heat of
        peppers by mixing pepper extract with a sugar-water solution
        such that the tongue experiences no sensation when tasting the
        mixture. The scoville-rating indicates the ratio of sugar
        solution to pepper needed for this test.]

  Extend grammar and reduction rules.

  >> DESIGN RECIPE << for conditionals
    draw intervals, 
    choose interior, boundary points as examples
    write conditional with conditions
    write down answers, case by case 

-------------------------------------------------------------------  

  Structures: 

  positions of a ball: x and y coordinate 
  lots of balls flying around in a game - better keep the x and y
  coordinates together
 
  posn structure 
  (make-posn 3 4)   - box diagram
  distance-to-O: oops doesn't work, need selectors : posn-x posn-y.

  do contracts for posn-x, posn-y, show examples of use.

  [The confusion over values will likely arise here, as subsequent
   examples appear to "create" the same value twice.

   Emphasize that "(make-posn 1 2)" is just the name of a position,
   like "7" is the name of a number. We can write "7" on the twice,
   but it's the same number.]

  Extend grammar. Note that (make-posn val val) shows up in the grammar
  for values. (make-posn expr expr) also shows up in the grammar for
  expressions, though.

  Example: ;shift-posn : posn num num -> posn
           ;moves a posn by the given x and y deltas
           (define (shift-posn a-posn delta-x delta-y) ...)

  >> DESIGN RECIPE (p69) <<