SICP_2.40

 1 #lang racket
 2 
 3 ;;;;;;;;;;;;;;;;;2.40
 4 (define nil ‘())
 5 
 6 (define (accumulate op intial seq)
 7   (if (null? seq)
 8       intial
 9       (op (car seq)
10           (accumulate op intial (cdr seq)))))
11 
12 (define (enumerate-interval low high)
13   (if (> low high)
14       nil
15       (cons low (enumerate-interval (+ low 1) high))))
16 
17 (define (make-pair-sum pair)
18   (list (car pair) (cadr pair) (+ (car pair) (cadr pair))))
19 
20 (define (prime-sum? pair)
21   (prime? (+ (car pair) (cadr pair))))
22 
23 (define (prime? n)
24   (define (test number)
25     (cond ((> (square number) n) #t)
26           ((= (remainder n number) 0) #f)
27           (else (test (+ number 1)))))
28   (test 2))
29 
30 (define (square x)
31   (* x x))
32 
33 (define (unique-pairs n)
34   (accumulate append
35               nil
36               (map (lambda (i)
37                      (map (lambda (j) (list i j))
38                           (enumerate-interval 1 (- i 1))))
39                    (enumerate-interval 1 n))))
40 
41 ;;;;;;;;test
42 (unique-pairs 5)
43 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
44 (define (prime-sum-pairs n)
45   (map make-pair-sum
46        (filter prime-sum?
47                (unique-pairs n))))
48 
49 ;;;;;;;;;test
50 (prime-sum-pairs 5)