Ruby difference in array including duplicates

Question

[1,2,3,3] - [1,2,3] produces the empty array []. Is it possible to retain duplicates so it returns [3]?

Answer 1

I am so glad you asked. I would like to see such a method added to the class Array in some future version of Ruby, as I have found many uses for it:

class Array
  def difference(other)
    h = other.each_with_object(Hash.new(0)) { |e,h| h[e] += 1 }
    reject { |e| h[e] > 0 && h[e] -= 1 }
  end
end

A description of the method and links to some of its applications are given here.

By way of example:

a = [1,2,3,4,3,2,4,2]
b = [2,3,4,4,4]

a - b          #=> [1]
a.difference b #=> [1,2,3,2]

Ruby v2.7 gave us the method Enumerable#tally, allowing us to replace the first line of the method with

h = other.tally