Getting all combinations of pairs from a list in Ruby

Question

I have a list of elements (e.g. numbers) and I want to retrieve a list of all possible pairs. How can I do that using Ruby?

Example:

l1 = [1,2,3,4,5] -> l2 = [[1,2][1,3][1,4][1,5][2,3][2,4][2,5][3,4][3,5][4,5]]

Answer 1

In Ruby 1.8.6, you can use Facets:

require 'facets/array/combination'
i1 = [1,2,3,4,5]
i2 = []
i1.combination(2).to_a # => [[1, 2], [1, 3], [1, 4], [1, 5], [2, 3], [2, 4], [2, 5], [3, 4], [3, 5], [4, 5]]

In 1.8.7 and later, combination is built-in:

i1 = [1,2,3,4,5]
i2 = i1.combination(2).to_a

Answer 2

Or, if you really want a non-library answer:

i1 = [1,2,3,4,5]
i2 = (0...(i1.size-1)).inject([]) {|pairs,x| pairs += ((x+1)...i1.size).map {|y| [i1[x],i1[y]]}}