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  1. ;; Pythagorean triples using combinations.
  2.  
  3. (use-modules (srfi srfi-1))
  4.  
  5. (define (combinations sequence k yield)
  6.   (let loop ((rest sequence) (n k) (out '()))
  7.     (if (zero? n)
  8.         (yield (reverse out))
  9.         (pair-for-each (lambda (p)
  10.                          (loop (cdr p) (- n 1) (cons (car p) out)))
  11.                        rest))))
  12.  
  13. (define (primitive-pythagorean-triple? x y z)
  14.   (and (< x y z)
  15.        (= (+ (* x x) (* y y)) (* z z))
  16.        (= (gcd x y z) 1)))
  17.  
  18. (define (pythagorean-triples n)
  19.   (let ((out '()))
  20.     (combinations (iota n 1) 3
  21.       (lambda (v)
  22.         (apply (lambda (x y z)
  23.                  (when (primitive-pythagorean-triple? x y z)
  24.                        (set! out (cons v out))))
  25.                v)))
  26.     (sort out (lambda (x y) (< (third x) (third y))))))
  27.  
  28. (for-each (lambda (x) (format #t "~A~%" x)) (pythagorean-triples 53))
Success #stdin #stdout 0.12s 12032KB
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(3 4 5)
(5 12 13)
(8 15 17)
(7 24 25)
(20 21 29)
(12 35 37)
(9 40 41)
(28 45 53)
https://ideone.com/KYKh3T
language:
Scheme (guile 2.2.4)
created:
7 days ago
visibility:
public

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