From: muraken@...
Date: 2018-03-08T09:26:28+00:00
Subject: [ruby-core:86040] [Ruby trunk Feature#4907][Closed] enumerable#permutation and combination

Issue #4907 has been updated by mrkn (Kenta Murata).

Status changed from Assigned to Closed

No use case is shown, so I close this issue.

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Feature #4907: enumerable#permutation and combination
https://bugs.ruby-lang.org/issues/4907#change-70902

* Author: neleai (Ondrej Bilka)
* Status: Closed
* Priority: Normal
* Assignee: mrkn (Kenta Murata)
* Target version: 
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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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