Some useful array methods.

array.rb

# Add some useful methods to the Ruby Array class.
# For Rails, add this class into the "initializers" folder:
# config/initializers/array.rb
class Array
  # Group the array's elements.
  #
  # Arguments:
  #  r: (Integer)
  #    - Max amount of elements each array's subgroup will have.
  def arrange(r)
    array, last, i = [], r, (size.to_f / r.to_f).ceil
    i.times do |k|
      array << self[(r * k)..(last - 1)]
      last += r
    end
    array
  end

  # Filter array by the even numbers.
  def even
    select { |e| e.is_a?(Integer) && e.even? }
  end

  # Filter array by the even indexes.
  def even_indexes
    values_at(* each_index.select(&:even?))
  end

  # Filter array by the odd numbers.
  def odd
    select { |e| e.is_a?(Integer) && e.odd? }
  end

  # Filter array by the odd indexes.
  def odd_indexes
    values_at(* each_index.select(&:odd?))
  end
end

#
# Examples
#

# Our awesome array...
foo = [-2, -1, 0, 1, 2, 3, 'a', :b, /regex/, BasicObject, 4.0, 5.0]

# Arrange it into groups of 4 elements.
p foo.arrange(4)

# Select even numbers.
p foo.even

# Select elements at even indexes.
p foo.even_indexes

# Select odd numbers.
p foo.odd

# Select elements at odd indexes.
p foo.odd_indexes