SICP 习题1.23

来源：互联网发布：生也有涯知也无涯编辑：程序博客网时间：2024/06/13 22:33

代码有点乱，题目中要求的next我是用的next-iter，因为不知不觉中我把next这个词给用了

开始把(define (next-iter …))写在了search-for-primes后面，怎么都不对。写在前面就对了，是因为search-for-primes需要在调用之前找到next-iter,是不会向后找的。（说的不好，大致就是这意思）

(define (smallest-divisor n)  (find-divisor n 2))(define (find-divisor n test-divisor)  (cond ((> (square test-divisor) n) n)        ((divides? test-divisor n) test-divisor)        (else (find-divisor n (next-iter test-divisor)))))(define (divides? a b)  (= (remainder b a) 0))(define (prime? n)  (= n (smallest-divisor n)))(define (square x) (* x x))(define (timed-prime-test n)  (newline)  (display n)  (start-prime-test n (runtime)))(define (start-prime-test n start-time)  (if (prime? n)    (report-prime (- (runtime) start-time))))(define (report-prime elapsed-time)  (display "***")  (display elapsed-time))(define (even? x)  (= (remainder x 2) 0))(define (next x)  (cond ((even? x) (+ x 1))        (else x)))(define (search-for-primes now tot-num)  (timed-prime-test (next now))  (cond ((= tot-num 0) #t)        ((and (prime? (next now)) (= tot-num 1)) #t)         ((prime? (next now)) (search-for-primes (+ (next now) 2) (- tot-num 1)))        (else (search-for-primes (+ (next now) 2) tot-num))))(define (next-iter i)  (cond ((= i 2) 3)        (else (+ i 2))))(search-for-primes 100 3);寻找大于某个数的三个素数

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