[plt-scheme] Moving-ball question - My question is when ball disappeared, instantly it comes to center of window

On Oct 28, 2009, at 5:11 PM, rahman usta wrote:

> My question is when ball disappeared, instantly it comes to center  
> of window

Not in my drscheme. The ball disappears and the program stops.

You may wish to add a call to (stop)

(define (move-ball-until-gone b)

   (cond
     ((out-of-bounds? b) (stop))
     (else (and
            (draw-and-clear-ball b)
            (move-ball-until-gone (move-ball b))))))


>
> ;; Teachpack: draw.ss
>
> (define-struct ball (radius x y x-dot y-dot color))
> ;; A Ball is (make-ball Number Number Number Number Number Symbol)
>
> #| Template
> 1. Trivial case:
> Ball is entirely out of bounds.  In that case, simply return true.
> 2. Divide:
> Draw the ball, erase it, then replace it by a moved copy.
> 3 Conquer:
> Nothing to do here.
> |#
>
> ;; move-ball-until-gone : Ball -> true
> ;; display a ball moving until it disappears from the window
> ;; uses generative recursion
> (define (move-ball-until-gone b)
>
>   (cond
>     ((out-of-bounds? b) true )
>         (else (and
>            (draw-and-clear-ball b)
>            (move-ball-until-gone (move-ball b))))))
>
> (define DELTA-T 1)      ; controls speed of simulation
> (define DELAY 0.1)      ; seconds between updates
>
> ;; move-ball : Ball -> Ball
> ;; models the movement of a ball
> (define (move-ball b)
>   (make-ball (ball-radius b)
>              (+ (ball-x b) (* DELTA-T (ball-x-dot b)))
>              (+ (ball-y b) (* DELTA-T (ball-y-dot b)))
>              (ball-x-dot b)
>              (ball-y-dot b)
>              (ball-color b)))
>
> ;; drawing window dimensions
> (define WIDTH 300)
> (define HEIGHT 300)
>
> ;; draw-and-clear-ball : Ball -> true
> ;; draws ball, waits a short time, then clears it
> (define (draw-and-clear-ball b)
>   (and (draw-solid-disk (make-posn (ball-x b) (ball-y b))
>                         (ball-radius b)
>                         (ball-color b))
>        (sleep-for-a-while DELAY)
>        (clear-solid-disk (make-posn (ball-x b) (ball-y b))
>                          (ball-radius b)
>                          (ball-color b))))
>
> ;; out-of-bounds? : Ball -> Boolean
> ;; determines if the ball is completely outside the drawing window
> (define (out-of-bounds? b)
>
>   {local [(define ball-left (- (ball-x b) (ball-radius b)))
>           (define ball-right (+ (ball-x b) (ball-radius b)))
>           (define ball-top (- (ball-y b) (ball-radius b)))
>           (define ball-bottom (+ (ball-y b) (ball-radius b)))]
>     (or (< ball-right 0) (> ball-left WIDTH)
>         (< ball-bottom 0) (> ball-top HEIGHT))})
>
> ;; test the program
> (start WIDTH HEIGHT)
> (move-ball-until-gone
>  (make-ball 10 50 50 3 0 'red))
>
>
>
>
> _________________________________________________
>  For list-related administrative tasks:
>  http://list.cs.brown.edu/mailman/listinfo/plt-scheme