SICP:反复用加法来实现乘法 1.17

#lang racket

(define (fast-multiplication a b n);a*n
  (cond ((= n 0) b);n==0
    ((even? n)
     (fast-multiplication (double a)
                  b
                  (halve n)));even? n
    (else
     (fast-multiplication a
                  (+ a b)
                  (- n 1))
     );else

    );cond
  );fast-multiplication  it‘s iteration

(define (fast-multiplication-new a n)
  (cond ((= n 0) 0)
    ((= n 1) a)
    ((even? n) (double (fast-multiplication-new a
                        (halve n))))

    (else (a +
         (fast-multiplication-new a
                      (- n 1)))
     );else
   );cond
  );fast-multiplication-new it‘s recursive

(define (double x)
  (+ x x));double

(define (halve x)
  (/ x
     2)
  );halve

(define (even? x)
  (= (remainder x
        2)
    0);=
  );even

(fast-multiplication-new 8 0 1)
(fast-multiplication-new 8 0 2)
(fast-multiplication-new 8 0 3)
(fast-multiplication-new 8 0 4)
(fast-multiplication-new 8 0 9)