Forum: Ruby array permutations

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Alex C. (Guest)
on 2007-07-23 08:51
The other day I ran into a problem where I needed all the permutations
of a given array. I knew I had covered this in some of my CS classes but
couldn't come up with the algorithm at first. I figured it out on the
drive home from work and decided to rubify it and add it as a method to
the array class. This is what I came up with and was just wondering if
anyone else had a cleaner or more effecient way of accomplishing this.

class Array
  def each_perm
    if self.size == 1
      yield self
    else
      self.each_index do |i|
        tmp, e = self.dup, self[i]
        tmp.delete_at(i)
        tmp.each_perm do |x|
          yield e.to_a + x
        end
      end
    end
  end
end


--AC
Dan Z. (Guest)
on 2007-07-23 10:37
(Received via mailing list)
Alex C. wrote:
>       yield self
> end
>
>
> --AC

Have a look at this thread:
http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/... .
It's a port of the GCC library version to ruby. As for efficiency,
you'll have to run them both (I haven't read either version carefully).
One factor that might make his slower is that it should avoids yielding
duplicates.

Have fun,
Dan
Raf C. (Guest)
on 2007-07-23 10:44
(Received via mailing list)
2007/7/23, Alex C. <removed_email_address@domain.invalid>:
>     if self.size == 1
>   end
> end
>
>
> --AC
> --
> Posted via http://www.ruby-forum.com/.
>
>

There's a problem with the "to_a" in "yield e.to_a + x". It gets a lot
of
"default `to_a' will be obsolete" warnings, and it gives the wrong
result
for, for instance, [1, 2, 'a'..'z'].each_perm. I'd go with "yield [e] +
x"
instead.

For the rest, looks OK to me.


Regards,
Raf
Ryan D. (Guest)
on 2007-07-23 10:49
(Received via mailing list)
On Jul 22, 2007, at 21:52 , Alex C. wrote:

> The other day I ran into a problem where I needed all the permutations
> of a given array. I knew I had covered this in some of my CS
> classes but
> couldn't come up with the algorithm at first. I figured it out on the
> drive home from work and decided to rubify it and add it as a
> method to
> the array class. This is what I came up with and was just wondering if
> anyone else had a cleaner or more effecient way of accomplishing this.

The one thing I learned in Algs is someone has invented/implemented/
tested it before me:

  http://rubygarden.org/Ruby/page/show/ArrayPermute

Please use google.
Ken B. (Guest)
on 2007-07-26 02:45
(Received via mailing list)
On Mon, 23 Jul 2007 15:36:24 +0900, Dan Z. wrote:

>>   def each_perm
>>     end
> One factor that might make his slower is that it should avoids yielding
> duplicates.

There is a version of each_permutation in the Facets library (http://
facets.rubyforge.org/) which is similar to the version you posted. After
relying on their version for several Ruby Q.zes, I decided that it
wasn't as useful to me as I would have liked, because I found I had to
do
bookkeeping for duplicates anyway, so I ported over the C++ STL version
which doesn't generate duplicates (because it's stateless).

Different design goals.

--Ken
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