Some histogram-related methods for ruby Enumerable types.

enumerable_histogram.rb

module Enumerable
  # Return a Hash whose keys are the unique elements and whose values are the number of appearences of that element.
  # [2,1,2,3,4,1,2,4,9,1,3,16,8].histogram => {2=>3, 1=>3, 3=>2, 4=>2, 9=>1, 16=>1, 8=>1}
  def histogram
    inject(Hash.new(0)) { |hash, x| hash[x] += 1; hash }
  end

  # Return an enumerable with the same elements sorted from the most frequent to least frequent.
  # See: http://codeidol.com/other/rubyckbk/Arrays/Sorting-an-Array-by-Frequency-of-Appearance/
  # [2,1,2,3,4,1,2,4,9,1,3,16,8].sort_by_frequency => [2, 2, 2, 1, 1, 1, 3, 3, 4, 4, 9, 16, 8]
  def sort_by_frequency
    h = histogram
    sort_by { |x| [h[x] * -1, index(x)] }
  end

  # Return an enumerable with uniq elements sorted from the most frequent to least frequent.
  # [2,1,2,3,4,1,2,4,9,1,3,16,8].uniq_by_frequency => [2, 1, 3, 4, 9, 16, 8]
  # Note: This is much faster than calling sort_by_frequency.uniq
  def uniq_by_frequency
    h = histogram
    uniq.sort_by { |x| [h[x] * -1, index(x)] }
  end
  
  # Return one from among the most frequent elements.
  # [2,1,2,3,4,1,2,4,9,1,3,16,8].most_frequent_element => 2
  def most_frequent_element
    uniq_by_frequency.first
  end
end