From: Aldric Giacomoni <aldric@...>
Date: 2009-12-19T01:50:13+09:00
Subject: Re: Unique combination of values from arrays

Giampiero Zanchi wrote:
> one = ['one', 'two']
> two = ['three', 'four']
> three = ['five', 'six']
> p one.product(two.product(three)).map {|x| x.flatten.join(" ")}
> 
> produces
> 
> ["one three five", "one three six", "one four five", "one four six", 
> "two three five", "two three six", "two four five", "two four six"]

That's really cool.
So...

def fancy_array_permutation arrays
  return [""] if arrays.empty?
  first = arrays.shift
  return first.product( fancy_array_permutation(arrays) ).map {|x| 
x.flatten.join(" ")}
end

produces

["one three five ", "one three six ", "one four five ", "one four six ",
"two three five ", "two three six ", "two four five ", "two four six "]

Blast. I did find this though:
http://snipplr.com/view/5087/arrayproduct-for-ruby-18/
    Unfortunately, this method uses too much memory, storing the whole 
cartesian product Array in memory even its elements are needed only one 
at a time. One nicer alternative is the Cartesian module:

    "The Cartesian module provide methods for the calculation of the 
cartesian producted between two enumberable objects. It can also be 
easily mixed in into any enumberable class, i.e. any class with 
Enumerable module mixed in."

    The Cartesian module is available at 
http://rubyforge.org/projects/cartesian/

This being said, a quick benchmark indicates that your code indeed runs 
exponentially faster than mine.. As indicated, probably at the expense 
of memory.
-- 
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