From: Trans <transfire@...>
Date: 2007-01-30T00:09:15+09:00
Subject: Re: cartesian product of arrays



On Jan 26, 6:05 am, Thomas Hafner <tho...@hafner.NL.EU.ORG> wrote:
> Hello,
>
> did I miss some standard library function? Anyway, then I'll do it on
> my own:
>
> def cartprod(*args)
>   result = [[]]
>   while [] != args
>     t, result = result, []
>     b, *args = args
>     t.each do |a|
>       b.each do |n|
>         result << a + [n]
>       end
>     end
>   end
>   result
> end
>
> Example:
> cartprod([1,2],[3,4,5],[6,7,8])
> => [[1, 3, 6], [1, 3, 7], [1, 3, 8], [1, 4, 6], [1, 4, 7], [1, 4, 8],
>     [1, 5, 6], [1, 5, 7], [1, 5, 8], [2, 3, 6], [2, 3, 7], [2, 3, 8],
>     [2, 4, 6], [2, 4, 7], [2, 4, 8], [2, 5, 6], [2, 5, 7], [2, 5, 8]]

Hi--

I was going to say that Facets has cross:

  require 'facets/core/enumerable/cross'

  Enumerable.cross([1,2],[3,4,5],[6,7,8])
  => [[1, 3, 6], [1, 3, 7], [1, 3, 8], [1, 4, 6], [1, 4, 7], [1, 4, 
8], [1, 5, 6], [1, 5, 7], [1, 5, 8], [2, 3, 6], [2, 3, 7], [2, 3, 8], 
[2, 4, 6], [2, 4, 7], [2, 4, 8], [2, 5, 6], [2, 5, 7], [2, 5, 8]]

Also when just deailing with a pair:

  require 'facets/core/enumerable/op_pow'

  [1,2] ** [3,4,5]
  => [[1, 3], [1, 4], [1, 5], [2, 3], [2, 4], [2, 5]]

there are two considerations though: 1) your implementation retains an 
additional array level for each initial possibility. is that desired 
behavior? and 2) Facets' implementation is poorly named conveying the 
cross product, which it is not, so unless someone has an objection, 
I'll rename it to #cart and credit you.

t.

(http://facets.rubyforge.org)