SICP_2.40

#lang racket

;;;;;;;;;;;;;;;;;2.40
(define nil '())

(define (accumulate op intial seq)
  (if (null? seq)
      intial
      (op (car seq)
          (accumulate op intial (cdr seq)))))

(define (enumerate-interval low high)
  (if (> low high)
      nil
      (cons low (enumerate-interval (+ low 1) high))))

(define (make-pair-sum pair)
  (list (car pair) (cadr pair) (+ (car pair) (cadr pair))))

(define (prime-sum? pair)
  (prime? (+ (car pair) (cadr pair))))

(define (prime? n)
  (define (test number)
    (cond ((> (square number) n) #t)
          ((= (remainder n number) 0) #f)
          (else (test (+ number 1)))))
  (test 2))

(define (square x)
  (* x x))

(define (unique-pairs n)
  (accumulate append
              nil
              (map (lambda (i)
                     (map (lambda (j) (list i j))
                          (enumerate-interval 1 (- i 1))))
                   (enumerate-interval 1 n))))

;;;;;;;;test
(unique-pairs 5)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
(define (prime-sum-pairs n)
  (map make-pair-sum
       (filter prime-sum?
               (unique-pairs n))))

;;;;;;;;;test
(prime-sum-pairs 5)