prime-test

secje posted @ 2010年5月10日 10:22 in sicp , 703 阅读

 

(define (square x) (* x x))
(define (fast-prime? n times)
  (define (expmod base exp m)
    (cond ((= exp 0) 1)
          ((even? exp)
           (remainder (square (expmod base (/ exp 2) m))
                      m))
          (else
           (remainder (* (remainder base m) (expmod base (- exp 1) m))
                    m))))
  (define (fermat-test n)
    (define (try-it a)
      (= (expmod a n n) a))
    (try-it (+ 1 (random (- n 1)))))
  (cond ((= times 0) true)
        ((fermat-test n) (fast-prime? n (- times 1)))
        (else false)))

PLT不认random,改用MIT Scheme了,看来要加快emacs的脚步了,越到后面谁知道还会发生什么,现在就是蛋疼的用PLT当编辑器和调试器,edwin做解释器

 

 

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