Scheme Turtle 5

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<language>Scheme</language>
===Random-Walk===

Diese Prozedur verdeutlich den Weg in den rekursiven Abstieg und zurück, wie er bei echter Rekursion stattfindet, grafisch.

<code hidden>
(define random-maker
  (let* ((multiplier 48271)
         (modulus 2147483647)
         (apply-congruence
          (lambda (current-seed)
            (let ((candidate (modulo (* current-seed multiplier)
                                     modulus)))
              (if (zero? candidate)
                  modulus
                  candidate))))
         (coerce
          (lambda (proposed-seed)
            (if (integer? proposed-seed)
                (- modulus (modulo proposed-seed modulus))
                19860617))))  ;; an arbitrarily chosen birthday
  (lambda (initial-seed)
    (let ((seed (coerce initial-seed)))
      (lambda args
        (cond ((null? args)
               (set! seed (apply-congruence seed))
               (/ (- modulus seed) modulus))
              ((null? (cdr args))
               (let* ((proposed-top
                       (ceiling (abs (car args))))
                      (exact-top
                       (if (inexact? proposed-top)
                           (inexact->exact proposed-top)
                           proposed-top))
                      (top
                       (if (zero? exact-top)
                           1
                           exact-top)))
                 (set! seed (apply-congruence seed))
                 (inexact->exact (floor (* top (/ seed modulus))))))
              ((eq? (cadr args) 'reset)
               (set! seed (coerce (car args))))
              (else
               (display "random: unrecognized message")
               (newline))))))))
</code>
<eval>
(define random
  (random-maker 19768116))  ;; Anfangswert verändern!

(clear canvas)
(move turtle 200 310)
(pencolor turtle 0 0 200)

(define random-walk
  (lambda (angle length n turtle)
    (if (= n 0)
        (penerase turtle)
        ;(pencolor turtle 220 0 0)
        (begin          
          (forward turtle length)
          (right turtle angle)
          (random-walk (+ 5 (random angle))(+ 5 (random length)) (- n 1) turtle)
          (left turtle angle)
          (backward turtle length)))))

(random-walk 360 20 100 turtle)
</eval>
<canvas h=500></canvas>

<code hidden>
;(define t (clone turtle)) 
;(hide t)
;(forward t 50)
;(right t 90)
;(forward t 50)
;(show t)
;(setangleto t turtle)
</code>