2.41.rkt

#lang sicp
;; Exercise 2.41:
;; Write a procedure to find all ordered triples of distinct positive integers i , j , and k less than or equal to a given integer n that sum to a given integer s .

(define (append list1 list2)
  (if (null? list1)
      list2
      (cons (car list1)
            (append (cdr list1)
                    list2))))

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

(define (add-to-s? pair s)
  (= s (+ (car pair) (cadr pair))))

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

(define (prime-sum-pairs n s)
  (map make-pair-sum
       (filter
        ()
        (flatmap
         (lambda (i)
           (map (lambda (j)
                  (list i j))
                (enumerate-interval
                 1
                 (- i 1))))
         (enumerate-interval 1 n)))))

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

(define (filter predicate sequence)
  (cond ((null? sequence) nil)
        ((predicate (car sequence))
         (cons (car sequence)
               (filter predicate
                       (cdr sequence))))
        (else  (filter predicate
                       (cdr sequence)))))