(modulo (+ position speed-for-one-step) WIDTH) evaluates to (+ position speed-for-one-step) if it is less than WIDTH and (- (+ position speed-for-one-step) WIDTH) if it is larger [roughly, but definitely in your case]
Does this help? On Mar 12, 2012, at 2:25 PM, ROELOF WOBBEN wrote: > Sorry, > > But I don't understand what the modulo and my solution work together. > > Roelof > > > On 12/03/12, Matthias Felleisen <[email protected]> wrote: >> >> >> #lang racket >> >> (require 2htdp/image 2htdp/universe) >> >> ; main : CarState -> CarState >> ; launch the program from some initial state >> (define (main r) >> (big-bang 0 (on-tick tock r) (to-draw render))) >> >> (define wheel-radius 5) >> (define cab >> (beside (rectangle (* 6 wheel-radius)(* 4 wheel-radius) "solid" "red") >> (right-triangle (* 2 wheel-radius) (* 4 wheel-radius) "solid" >> "red"))) >> (define body >> (rectangle (* 20 wheel-radius) (* 6 wheel-radius) "solid" "red")) >> (define top >> (above/align "right" cab body)) >> (define tire >> (circle wheel-radius "solid" "black")) >> (define bot >> (beside tire >> (rectangle (* 5 wheel-radius) (* 2 wheel-radius) "solid" "white") >> tire)) >> (define auto >> (above top bot)) >> >> (define WIDTH 1000) >> (define SPEED (* 0.54 (image-width auto))) >> (define Y-CAR 50) >> (define BACKGROUND (empty-scene WIDTH 200)) >> >> >> ; CarState -> CarState >> ; the clock ticked; move the car by three pixels >> ; example: >> ; given: 20, expected 23 >> ; given: 78, expected 81 >> (define (tock ws) >> (modulo (+ ws SPEED) WIDTH) >> #; >> (if (< ws (-(image-width BACKGROUND) (* 0.54 (image-width auto)))) >> (+ ws 3) >> ( - ws (image-width BACKGROUND)))) >> >> ; PositiveNumber -> Image >> ; run the animation at the specified rate [1/28 is decent, try 1/100 too] >> (define (render ws) >> (place-image auto ws Y-CAR BACKGROUND)) >> >> >> On Mar 12, 2012, at 10:24 AM, ROELOF WOBBEN wrote: >> >>> Hello, >>> >>> I have tried to make a function which display a car running on a >>> empty-screen. >>> When the car is at the end. The car has to begin at the beginning. >>> >>> I have made this : >>> >>> (define wheel-radius 5) >>> (define cab (beside (rectangle (* 6 wheel-radius)(* 4 wheel-radius) "solid" >>> "red")(right-triangle (* 2 wheel-radius) (* 4 wheel-radius) "solid" "red"))) >>> (define body (rectangle (* 20 wheel-radius) (* 6 wheel-radius) "solid" >>> "red")) >>> (define top (above/align "right" cab body)) >>> (define tire (circle wheel-radius "solid" "black")) >>> (define bot (beside tire (rectangle (* 5 wheel-radius) (* 2 wheel-radius) >>> "solid" "white") tire)) >>> (define auto (above top bot)) >>> >>> (define Y-CAR 50) >>> (define BACKGROUND (empty-scene 1000 200)) >>> >>> ; CarState -> CarState >>> ; the clock ticked; move the car by three pixels >>> ; example: >>> ; given: 20, expected 23 >>> ; given: 78, expected 81 >>> (define (tock ws) >>> (if (< ws (-(image-width BACKGROUND)(* 0.54 (image-width auto))))(+ ws >>> 3) ( - ws (image-width BACKGROUND)))) >>> >>> ; CarState -> Image >>> ; place the car into a scene, according to the given world state >>> (define (render ws) >>> (place-image auto ws Y-CAR BACKGROUND)) >>> >>> ; main : CarState -> CarState >>> ; launch the program from some initial state >>> (define (main ws) >>> (big-bang ws (on-tick tock) (to-draw render))) >>> >>> But now I wonder if I can do the same with setting the variable ws to zero. >>> So like this : If ws is smaller then the image-width increase with the half >>> width of the car. then ws = ws+ 3 else ws = 0 >>> >>> Regards, >>> >>> Roelof >>> >>> >>> >>> >>> >>> >>> >>> >>> >>> ____________________ >>> Racket Users list: >>> http://lists.racket-lang.org/users >> > ____________________ > Racket Users list: > http://lists.racket-lang.org/users
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