Cartesian product of array with itself without duplicates in Ruby

Question

How can I do a cartesian product of an array with itself, without including both [object i, object j] and [object j, object i]?

Currently, I've got

array = %w{a b c}
unique_combinations = array.each_with_index.to_a.product(array.each_with_index.to_a).
  find_all{|(first_object, i), (second_object, j)| i < j}.
  map{|(first_object, i), (second_object, j)| [first_object, second_object]}
unique_combinations # => [["a", "b"], ["a", "c"], ["b", "c"]]

which works, but feels a little verbose.

I could do

array = %w{a b c}
combinations = array.product(array)
unique_combinations = combinations.find_all{|first_item, second_item| array.index(first_item) < array.index(second_item)}

but that feels like I'm throwing information away, and would only work if the array only had unique items in it.

Another approach is

unique_combinations = []
array.each_with_index do |first_item, i|
  array.each_with_index do |second_item, j|
    next unless i < j
    unique_combinations << [first_item, second_item]
  end
end

but that feels too imperative rather than functional.

Casper · Accepted Answer · 2012-06-07 01:51:00Z

6

It's called a combination?

a = %w{a b c}

a.combination(2).to_a
=> [["a", "b"], ["a", "c"], ["b", "c"]]

edited Jun 7, 2012 at 1:51

answered Jun 7, 2012 at 1:44

Casper

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