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  1. ;; Generate combinations. (1.02)
  2.  
  3. (use-modules (srfi srfi-1))
  4.  
  5. ;; Utility.
  6.  
  7. (define (pair-map proc l)
  8.   (pair-fold-right (lambda (item init)
  9.                      (cons (proc item) init))
  10.                    '() l))
  11.  
  12. (define (pair-append-map proc l)
  13.   (apply append (pair-map proc l)))
  14.  
  15. ;; Combinations.
  16.  
  17. (define (combinations sequence k)
  18.   (if (zero? k)
  19.       (list '())
  20.       (pair-append-map (lambda (rest)
  21.                          (map (lambda (comb)
  22.                                 (cons (car rest) comb))
  23.                               (combinations (cdr rest) (- k 1))))
  24.                        sequence)))
  25.  
  26. ;; Show.
  27.  
  28. (define n 5)
  29. (define a (iota n 1))
  30. (for-each (lambda (k)
  31.             (format #t "~A choose ~A~%" n k)
  32.             (for-each (lambda (c)
  33.                         (format #t " ~A~%" c))
  34.                       (combinations a k)))
  35.           (iota n 1))
Success #stdin #stdout 0.02s 10936KB
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5 choose 1
 (1)
 (2)
 (3)
 (4)
 (5)
5 choose 2
 (1 2)
 (1 3)
 (1 4)
 (1 5)
 (2 3)
 (2 4)
 (2 5)
 (3 4)
 (3 5)
 (4 5)
5 choose 3
 (1 2 3)
 (1 2 4)
 (1 2 5)
 (1 3 4)
 (1 3 5)
 (1 4 5)
 (2 3 4)
 (2 3 5)
 (2 4 5)
 (3 4 5)
5 choose 4
 (1 2 3 4)
 (1 2 3 5)
 (1 2 4 5)
 (1 3 4 5)
 (2 3 4 5)
5 choose 5
 (1 2 3 4 5)
https://ideone.com/sGQo9z
language:
Scheme (guile 2.2.4)
created:
7 days ago
visibility:
public

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