combination.rb

class Hoge
  def combination(array, num)
    if array.empty? || num == 0
      []
    elsif num == 1
      array.map{ |elem| [elem] }
    else
      rest = array.drop(1)
      combination(rest, num - 1).map{ |elem| [array.first] + elem } +
        combination(rest, num)
    end
  end
end

array = %w(a b c d e f g)
p Hoge.new.combination(array, 4) == array.combination(4).to_a

combination.scm

(define (combination lst n)
  (cond ((or (null? lst)
             (= n 0))
         '())
        ((= n 1)
         (map (lambda (x) (list x)) lst))
        (else
         (append (map (lambda (x) (cons (car lst) x))
                      (combination (cdr lst) (- n 1)))
                 (combination (cdr lst) n)))))
               

test

gosh> (combination '() 1)
()
gosh> (combination '(a b c) 0)
()
gosh> (combination '(a b c) 1)
((a) (b) (c))
gosh> (combination '(a b c) 2)
((a b) (a c) (b c))
gosh> (combination '(a b c d e) 2)
((a b) (a c) (a d) (a e) (b c) (b d) (b e) (c d) (c e) (d e))
gosh> (combination '(a b c d e) 3)
((a b c) (a b d) (a b e) (a c d) (a c e) (a d e) (b c d) (b c e) (b d e) (c d e))
gosh> (combination '(a b c d e) 4)
((a b c d) (a b c e) (a b d e) (a c d e) (b c d e))
gosh> (combination '(a b c d e) 5)
((a b c d e))
gosh> (combination '(a b c d e) 6)
()