Determine if one array contains all elements of another array

Question

I need to tell if an array contains all of the elements of another array with duplicates.

[1,2,3].contains_all? [1,2]   #=> true
[1,2,3].contains_all         


        

Answer 1

Here's a naive and straightforward implementation (not the most efficient one, likely). Just count the elements and compare both elements and their occurrence counts.

class Array
  def contains_all? ary
    # group the arrays, so that 
    #   [2, 1, 1, 3] becomes {1 => 2, 2 => 1, 3 => 1}
    my_groups = group_and_count self
    their_groups = group_and_count ary

    their_groups.each do |el, cnt|
      if !my_groups[el] || my_groups[el] < cnt
        return false
      end
    end

    true
  end

  private
  def group_and_count ary
    ary.reduce({}) do |memo, el|
      memo[el] ||= 0
      memo[el] += 1
      memo
    end
  end

end

[1, 2, 3].contains_all? [1, 2]   # => true
[1, 2, 3].contains_all? [1, 2, 2] # => false
[2, 1, 2, 3].contains_all? [1, 2, 2] # => true
[1, 2, 3].contains_all? [] # => true
[].contains_all? [1, 2] # => false