[racket] 2htdp/image Sierpinski

I adapted the Sierpinski example from the racket-lang.org home page for
Intermediate Student Language. The parameter represents recursive depth,
which requires no division and a single local definition. If it weren't for
the local, this would be a BSL program:

(require 2htdp/image)

;; Examples
(define st0 (triangle 16 'solid 'red))
(define st1 (above st0 (beside st0 st0)))

;; Tests
(check-expect (ski 0) st0)
(check-expect (ski 1) st1)

;; number -> image
;; Draws Sierpinski Triangle to given recursive depth
(define (ski n)
  (cond
    [(zero? n) (triangle 16 'solid 'red)]
    [else (local ((define next (ski (- n 1))))
            (above next (beside next next)))]))

;; Draw Sierpinski Triangles to different recursive depths
(ski 0)
(ski 1)
(ski 2)
(ski 3)


Hope this helps,
Marc


Gregor Kiczales wrote:

> Date: Sat, 30 Oct 2010 21:18:43 -0700

From: Gregor Kiczales <gregor at cs.ubc.ca>

To: users at racket-lang.org

Subject: [racket] 2htdp/image Sierpinski

Message-ID: <EFC7275A-B06E-463F-AE96-EA9E0C36CCC7 at cs.ubc.ca>

Content-Type: text/plain; charset=us-ascii


> In 2htdp/image the implementations of Sierpinski triangle and carpet are
> nice and simple, which is great for using them in class.


>
> (define (carpet size c)

 (if (<= size 2)

     (square size "outline" c)

     (local [(define sub (carpet (/ size 3) c))

             (define ctr (square (/ size 3) "solid" "white"))]

       (overlay (square size "outline" c)

                (above (beside sub sub sub)

                       (beside sub ctr sub)

                       (beside sub sub sub))))))


> But...  (carpet 200 "red") produces some strange behavior on my Mac.
> (carpet 300 "red") is even worse.  The image draws very slowly, which I
> assume comes from the way the image is actually composed of all the composed
> images rather than a single bitmap. But, stranger than that, it seems to
> need to draw itself several times.


> Also I would have thought that 'forcing' the image by actually drawing it
> would save the bitmap so that future drawing was faster. But I may not
> understand how this is actually working.


> Is this style of using above, beside and friends to make something this big
> intended to work? Or is there a different style I should be using?
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