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  1. ;; Generate Pythagorean triples using combinations. (1.04)
  2. ;; @see RhRctK
  3.  
  4. (use-modules (srfi srfi-1)   ; pair-for-each
  5.              (srfi srfi-67)  ; list-compare
  6.              (ice-9 format)) ; advanced formatting
  7.  
  8. ;; Combinations.
  9.  
  10. (define (combinations sequence k yield)
  11.   (let loop ((start sequence) (k k) (result '()))
  12.     (if (zero? k)
  13.         (yield (reverse result))
  14.         (pair-for-each (lambda (rest)
  15.                          (loop (cdr rest) (- k 1) (cons (car rest) result)))
  16.                        start))))
  17.  
  18. ;; Pythagorean triples.
  19.  
  20. (define (primitive-pythagorean-triple? x y z)
  21.   (and (< x y z)
  22.        (= (+ (* x x) (* y y)) (* z z))
  23.        (= (gcd x y z) 1)))
  24.  
  25. (define (pythagorean-triples n)
  26.   (let* ((out '())
  27.          (collect (lambda (v)
  28.                     (apply (lambda (x y z)
  29.                              (when (primitive-pythagorean-triple? x y z)
  30.                                    (set! out (cons v out))))
  31.                            v))))
  32.     (combinations (iota n 1) 3 collect)
  33.     (reverse out)))
  34.  
  35. ;; Main.
  36.  
  37. (define (list-less-reverse x y)
  38.   (< (list-compare (reverse x) (reverse y)) 0))
  39.  
  40. (for-each (lambda (v)
  41.             (format #t "(~{~2D~^ ~})~%" v))
  42.           (sort (pythagorean-triples 97) list-less-reverse))
Success #stdin #stdout 0.53s 12692KB
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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)
(11 60 61)
(33 56 65)
(16 63 65)
(48 55 73)
(36 77 85)
(13 84 85)
(39 80 89)
(65 72 97)
https://ideone.com/Yn3fZb
language:
Scheme (guile 2.2.4)
created:
11 days ago
visibility:
public

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