From: Michael DeHaan <michael.dehaan@...>
Date: 2004-11-10T22:37:23+09:00
Subject: Re: Iterating trough hash

"each_ordered_pair_of_elements_in_the_set_except_for_equal_elements"

But that would just be overly silly... :)


On Wed, 10 Nov 2004 08:08:49 +0900, Mark Hubbart <discordantus@gmail.com> wrote:
> On Wed, 10 Nov 2004 06:51:50 +0900, Brian Schr�der
> 
> 
> <ruby@brian-schroeder.de> wrote:
> > On Wed, 10 Nov 2004 06:41:08 +0900
> >
> >
> > Michael DeHaan <michael.dehaan@gmail.com> wrote:
> >
> > > Hmm, that's not a good bug there, so I'll pull the "history" thing out
> > > (I was trying to be too clever)...
> > >
> > > module Enumerable
> > >  def each_permutation
> > >    each do |item1|
> > >      each do |item2|
> > >          yield [item1,item2] unless (item1 === item2)
> > >      end
> > >    end
> > >  end
> > > end
> > >
> > >
> >
> > Hello Michael,
> >
> > I think, that this is not each_permutation, but each_ordered_pair_of_elements_in_the_set_except_for_equal_elements.
> >
> > each_permutation would be all shuffles of the original enumerable.
> >
> > And it is not what the OP wanted, as he wanted all unordered pairs. So maybe
> 
> oops. I'll try to correct myself now...
> 
> The method I defined should be called each_combination_pair; they
> weren't permutations at all.
> 
> So, I submit a revised method:
> 
> module Enumerable
>   def combinations(min=0, max = nil)
>     size = 0
>     com = [[]]
>     each do |item1|
>       size += 1
>       com += com.map{|item2| item2.dup << item1 }
>     end
>     com.select{|n| (min..max||size) === n.size}
>   end
> end
> 
> Then:
> 
> h.combinations(2,2).each do |(person1, age1), (person2, age2)|
>   puts "#{person1} shakes hands with #{person2}." +
>     "Together they are #{age1+age2} years old."
> end
> 
> I'm sure #combinations could be optimized to not bother generating
> combinations of the wrong size, but it's already questionable whether
> the method is short to justify replacing the original code, which is
> probably more efficient :)
> 
> cheers,
> Mark
> 
>