<div style="font-family: 'Times New Roman'; font-size: 16px;">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 </div>