ps. Your program's light function does NOT map Strings to Images. It maps World to images, ASSUMING you have defined
;; World is ... (probably string here but 1, 2, and 3 would work and so would many others). On Mar 19, 2012, at 4:25 PM, Matthias Felleisen wrote: > > 1. Your program does NOT change a string. It changes the state of the traffic > light (or it tells drracket to keep track of a different state). The string > "rood" remains the same in eternity. > > 2. You're missing test cases, and if you had tested the program you would > have discovered it before evaluating (main "rood"). > > 3. You can understand your program in terms of interactions after it signals > an error: > >> (tock "rood") ;; what is the state after one clock tick > "groen" >> (tock (tock "rood")) ;; what is the state after two clock ticks > "oranje" >> (light (tock (tock "rood"))) ;; how will DrRacket render the state after two >> clock ticks > . ;; <-- some yellow circle >> ;; <-- now try three clock ticks. > > > > On Mar 19, 2012, at 3:10 PM, Roelof Wobben wrote: > >> Hello, >> >> I have to make a traffic-light which changes color after each tick of the >> computer. >> So I thought this would work : >> >> >> ;; String -> Image >> ;; given a state of a trafic-light display a circle of the given color if >> it's a color of the traffic-light otherwise error >> (define (light color) >> (cond >> [(string=? color "oranje") (circle 10 "solid" "yellow")] >> [(string=? color "rood") (circle 10 "solid" "red" )] >> [(string=? color "groen") (circle 10 "solid" "green" )] >> [else (error "Error : geen kleur van een verkeerslicht ingevoerd in de >> functie" " light")] >> )) >> >> ;; World -> World >> ;; Given a World and set the traffic-light to the next color >> (define (tock color) >> ( cond >> [ (string=? color "oranje") "red"] >> [ (string=? color "rood") "groen"] >> [ (string=? color "groen") "oranje"] >> )) >> >> (define (main color) >> ( big-bang color (on-tick tock) (to-draw light))) >> >> (main "rood") >> >> But the contents of color does not change so I get a error message. >> What is the racket way to change the value of a string. >> >> Roelof >> >> ____________________ >> Racket Users list: >> http://lists.racket-lang.org/users > > > ____________________ > Racket Users list: > http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
