09 - sorting an array by frequency of appearance.rb

O Reilly Ruby Cookbook source code

module Enumerable
  def sort_by_frequency
    histogram = inject(Hash.new(0)) { |hash, x| hash[x] += 1; hash}
    sort_by { |x| [histogram[x], x] }
  end
end

[1,2,3,4,1,2,4,8,1,4,9,16].sort_by_frequency
# => [3, 8, 9, 16, 2, 2, 1, 1, 1, 4, 4, 4]
#---
[1,2] <=> [0,2]                                     # => 1
[1,2] <=> [1,2]                                     # => 0
[1,2] <=> [2,2]                                     # => -1
[1,2] <=> [1,1]                                     # => 1
[1,2] <=> [1,3]                                     # => -1
[1,2] <=> [1]                                       # => 1
[1,2] <=> [3]                                       # => -1
[1,2] <=> [0,1,2]                                   # => 1
[1,2] <=> []                                        # => 1
#---
module Enumerable
  def sort_by_frequency_faster
    histogram = inject(Hash.new(0)) { |hash, x| hash[x] += 1; hash}
    sort_by { |x| histogram[x] }
  end
end

[1,2,3,4,1,2,4,8,1,4,9,16].sort_by_frequency_faster
# => [16, 8, 3, 9, 2, 2, 4, 1, 1, 4, 4, 1]
#---
module Enumerable
  def sort_by_frequency_descending
    histogram = inject(Hash.new(0)) { |hash, x| hash[x] += 1; hash}
    sort_by { |x| [histogram[x] * -1, x]}
  end
end

[1,2,3,4,1,2,4,8,1,4,9,16].sort_by_frequency_descending
# => [1, 1, 1, 4, 4, 4, 2, 2, 3, 8, 9, 16]
#---
module Enumerable
  def sort_distinct_by_frequency
    histogram = inject(Hash.new(0)) { |hash, x| hash[x] += 1; hash }
    histogram.keys.sort_by { |x| [histogram[x], x] }
  end
end

[1,2,3,4,1,2,4,8,1,4,9,16].sort_distinct_by_frequency
# => [3, 8, 9, 16, 2, 1, 4]
#---