yield_example.rb

# return every n'th element in an enumerable
module Enumerable
  def every_nth(count)
    index = 0
    self.each do |elt|
      yield elt if index % count == 0
      index += 1
    end
  end
end

list = (1..10).to_a # make an array from a range
list.every_nth(3) { |s| print "#{s}, " }
# => 1, 4, 7, 10,
list.every_nth(2) { |s| print "#{s}, " }
# => 1, 3, 5, 7, 9,