Feature #21564: Extend permutation, repeated_permutation, combination and repeated_combination argument

• Status: Open
• Assignee: -
• Target version: -

[toy (Ivan Kuchin)]

When using functions permutation, repeated_permutation, combination and repeated_combination, often one needs not one, but multiple permutation/combination sizes. Currently all functions accept one Integer argument (for permutation it is optional and defaults to array size), and it would be more powerful if it would accept also a range or a Enumerable:

a = [1, 2, 3]

a.permutation(1..3).to_a # => [[1], [2], [3],
                         #     [1, 2], [1, 3], [2, 1], [2, 3], [3, 1], [3, 2],
                         #     [1, 2, 3], [1, 3, 2], [2, 1, 3], [2, 3, 1], [3, 1, 2], [3, 2, 1]]

a.permutation([1, 3]).to_a # => [[1], [2], [3],
                           #     [1, 2, 3], [1, 3, 2], [2, 1, 3], [2, 3, 1], [3, 1, 2], [3, 2, 1]]

a.permutation([3, 1]).to_a # => [[1, 2, 3], [1, 3, 2], [2, 1, 3], [2, 3, 1], [3, 1, 2], [3, 2, 1],
                           #     [1], [2], [3]]

a.repeated_permutation([2, 1]).to_a # => [[1, 1], [1, 2], [1, 3], [2, 1], [2, 2], [2, 3], [3, 1], [3, 2], [3, 3],
                                    #     [1], [2], [3]]

a.combination(1..3).to_a # => [[1], [2], [3],
                         #     [1, 2], [1, 3], [2, 3],
                         #     [1, 2, 3]]

a.repeated_combination(1..3).to_a # => [[1], [2], [3],
                                  #     [1, 1], [1, 2], [1, 3], [2, 2], [2, 3], [3, 3],
                                  #     [1, 1, 1], [1, 1, 2], [1, 1, 3], [1, 2, 2], [1, 2, 3], [1, 3, 3], [2, 2, 2], [2, 2, 3], [2, 3, 3], [3, 3, 3]]

Ruby code to enhance current permutation would be:

class Array
  alias_method :original_permutation, :permutation

  def permutation(counts, &block)
    return to_enum(__method__, counts) unless block

    if counts.is_a?(Enumerable)
      counts.each do |count|
        original_permutation(count, &block)
      end
    else
      original_permutation(counts, &block)
    end
  end
end

[nobu (Nobuyoshi Nakada)]

I'm not sure whether this is a natural extension of permutation.

And why not array.permutation(count1, count2, count3)?
Is it important to support Range without splatting?

[mame (Yusuke Endoh)]

When proposing new features, I strongly recommend writing a use case. It should be as simple and relatable as possible, but if that is difficult, a concrete example that you are actually facing now and that demonstrates general applicability is acceptable. Appealing to conceptual benefits like "powerful" is weak.