2-40.rkt

#lang racket

(define (inc x) (+ 1 x))
(define (square x) (* x x))

(define (divides? a b) (= (remainder b a) 0))

(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 (+ test-divisor 1)))))

(define (prime? n)
  (= n (smallest-divisor n)))

(define (accumulate op initial sequence)
  (if (null? sequence)
      initial
      (op (car sequence)
          (accumulate op initial (cdr sequence)))))

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

(define n 10)

(accumulate append null (map (lambda (i)
                               (map (lambda (j) (list i j))
                                    (enumerate-interval 1 (- i 1))))
                               (enumerate-interval 1 n)))

(define (flatmap proc seq)
  (accumulate append null (map proc seq)))

(define (unique-pairs n)
  (flatmap (lambda (i)
             (map (lambda (j) (list i j))
                  (enumerate-interval 1 (- i 1))))
           (enumerate-interval 1 n)))

(unique-pairs n)

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

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

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

(prime-sum-pairs n)