Issue #4907 has been reported by Ondrej Bilka.

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Feature #4907: enumerable#permutation and combination
http://redmine.ruby-lang.org/issues/4907

Author: Ondrej Bilka
Status: Open
Priority: Normal
Assignee: 
Category: 
Target version: 


Hello
Methods permutation and combination are defined for array but it make more sense to define them for enumerable. 
Here is sample implementation which for simplicity works only with blocks.
Note that implementation works lazily. 
It changes traversal order but documentation states that order is unspecified.

module Enumerable
 def perm(n)
 comb(n){|ary|
 ary.permutation{|p| yield(p)}
 }
 end
 def comb(n,&m)
 ary=[]
 e=to_enum.with_index
 _comb(e,n-1,1.0/0.0,ary,m)
 end
def _comb(e,n,bound,ary,m)
    e.each{|el,i|
      return if i>=bound
      ary[n]=el
      if n==0
        m.call(ary)
      else
        _comb(e,n-1,i,ary,m)
      end
    }
  end

end
[1,2,4,3,5].comb(2){|a| puts a.inspect}
(1..4).comb(2){|a| puts a.inspect}
(1..4).perm(2){|a| puts a.inspect}



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