; CS 111 - Week 4 Lecture 2 - 2016-09-15
; Sharon Tuttle
; last modified: 2016-09-15
(require 2htdp/image)
(require 2htdp/universe)
; cond expression syntax:
;
; (cond
; [boolean_expr1 expression1]
; [boolean_expr2 expression2]
; ...
; [else expression-else
; )
;
; semantics: DrRacket tries each
; boolean expression UNTIL it finds
; one that's #true -- when it does,
; that clause's 2nd expression IS the
; value/result for this cond expression
; (if none? then the else's expression
; becomes the cond expression's value)
; (I *think* BSL Racket will let you omit
; the else clause, BUT will give an
; error if it is NEEDED and not there...)
;========
; another branching example:
; in Math, you might recall:
; abs-value(x) = x if x >=0
; -x if x < 0
; signature: abs-value: number -> number
; purpose: expects a number, returns the
; absolute value of that number:
; abs-value(x) = x if x >=0
; -x if x < 0
;(define (abs-value num)
; ...
;)
; TWO categories of interval-style data,
; with ONE boundary -- so at least THREE
; tests are needed;
(check-expect (abs-value 11)
11)
(check-expect (abs-value -3)
3)
(check-expect (abs-value 0)
0)
; I have TWO categories of data --
; I will likely have at least TWO clauses,
; two BRANCHES if you will, in my cond
;(define (abs-value num)
; (cond
; [... ...]
; [... ...]
; )
;)
(define (abs-value num)
(cond
[(>= num 0) num]
[else (- 0 num)]
)
)
;========
; back to the penguin example!
(define FLOATING-PENGUIN 
)
(define LANDED-PENGUIN 
)
(define WIDTH 200)
(define HEIGHT 250)
(define BACKDROP (empty-scene WIDTH HEIGHT))
; if I want an animation with these penguins,
; using big-bang,
; I need a function that expects a world value
; and returns the "right" penguin-scene for
; that world value;
; signature: draw-penguin: number -> scene
; purpose: expects a world-number,
; and if the world number is less than
; (HEIGHT - 50), then this returns a scene of
; a floating penguin centered at a y coordinate
; of that world value;
; otherwise, this returns a scene of a landed
; penguin 50 pixels above the "ground"
; (at the bottom of the
; scene)
;(define (draw-penguin world-num)
; ...
;)
; need at least 3 tests -- one for
; floating, one for landed, one for
; boundary...
(check-expect (draw-penguin 10)
(place-image FLOATING-PENGUIN
(/ WIDTH 2)
10
BACKDROP))
(check-expect (draw-penguin (+ HEIGHT 1000))
(place-image LANDED-PENGUIN
(/ WIDTH 2)
(- HEIGHT 50)
BACKDROP))
(check-expect (draw-penguin (- HEIGHT 50))
(place-image LANDED-PENGUIN
(/ WIDTH 2)
(- HEIGHT 50)
BACKDROP))
;(define (draw-penguin world-num)
; (cond
; [... ...]
; [... ...]
; )
;)
(define (draw-penguin world-num)
(cond
[(< world-num (- HEIGHT 50))
(place-image FLOATING-PENGUIN
(/ WIDTH 2)
world-num
BACKDROP)]
[else
(place-image LANDED-PENGUIN
(/ WIDTH 2)
(- HEIGHT 50)
BACKDROP)]
)
)
;========
; an INTERVAL is a kind of data where
; branching is useful
; * you have a description of a set of numbers
; described by its boundaries
; * you OFTEN have a clause/branch for each
; interval;
; * you at least need to have a TEST for each
; interval, and for each boundary between
; intervals (and CAN add additional tests as
; desired!)
;=========
; another useful category of data where branching
; is useful:
; * enumerations
; * when your data is relatively small
; number of discrete, specific cases
; * a traffic light can be yellow, green, red
; * arrow keys on a computer are often
; up, down, left, right
; * some multiple-choice tests have questions
; whose answers are 1, 2, 3, or 4
; really, these are an informal "little" special
; kind of data --
; we are going to make a DATA DEFINITION COMMENT
; to make this clear
; (We'll write the names of these "new" data types
; in CamelCase, to make clear they are "new",
; but still data types)
; DATA DEFINITION:
; an AnimalSound is one of:
; "moo", "baa", "la la la", "oink"
; (inspired by Sandra Boynton's "Moo, Baa, La La La")
; signature: next-sound: AnimalSound -> AnimalSound
; purpose: expects an animal sound, (as defined above),
; and returns the "next" animal sound I should hear
; based on the the beginning of
; Sandra Boynton's "Moo, Baa, La La La":
; * "moo"? -> "baa"
; * "baa"? -> "la la la"
; * "la la la"? -> "oink"
; * "oink"? -> "moo"
; (if they give a NON animal sound, return "???")
;(define (next-sound an-animal-sound)
; ...
;)
; when writing tests for functions involving
; enumerations,
; you usually need a test for EACH possible value of
; the enumeration, and often one more for
; if the user gives a "bad" value;
(check-expect (next-sound "moo")
"baa")
(check-expect (next-sound "baa")
"la la la")
(check-expect (next-sound "la la la")
"oink")
(check-expect (next-sound "oink")
"moo")
(check-expect (next-sound "arf")
"???")
(big-bang 0
(to-draw draw-penguin)
(on-tick add1))
; for enumerations,
; you often have a branch/clause for each possible
; value, plus one for "bad" values;
;(define (next-sound an-animal-sound)
; (cond
; [... ...]
; [... ...]
; [... ...]
; [... ...]
; [... ...]
; )
;)
(define (next-sound an-animal-sound)
(cond
[(string=? an-animal-sound "moo") "baa"]
[(string=? an-animal-sound "baa") "la la la"]
[(string=? an-animal-sound "la la la") "oink"]
[(string=? an-animal-sound "oink") "moo"]
[else "???"]
)
)
(next-sound "moo")
; what if big-bang had a string world instead of a number?
; I might want:
; signature: draw-string-world: string -> scene
; purpose: expects a world-string, and returns
; a scene with that world-string in its center,
; in blue 40-point letters
;(define (draw-string-world world-string)
; ...
;)
(check-expect (draw-string-world "Hi!")
(place-image
(text "Hi!" 40 "blue")
(/ WIDTH 2)
(/ HEIGHT 2)
BACKDROP))
(check-expect (draw-string-world "Lo!")
(place-image
(text "Lo!" 40 "blue")
(/ WIDTH 2)
(/ HEIGHT 2)
BACKDROP))
(define (draw-string-world world-string)
(place-image
(text world-string 40 "blue")
(/ WIDTH 2)
(/ HEIGHT 2)
BACKDROP)
)
; let's use draw-string-world and next-sound in
; a big-bang expression...
; (note: on-tick clause has an optional 2nd argument,
; how often (in seconds) to tick the ticker;
; by default it tries to tick it every 1/28 second,
; here I am asking it to tick every 1 second...)
(big-bang "moo"
(to-draw draw-string-world)
(on-tick next-sound 1))