trafficlight.rb

class TrafficLight
  def initialize
    @colors = ["green", "yellow", "red"]
    @index = 0
  end
  
  def color
    @colors[@index]
  end
  
  def cycle!
    @index = (@index + 1) % @colors.size
  end
end

light = TrafficLight.new
light.color                     # => "green"
light.cycle!
light.color                     # => "yellow"
light.cycle!
light.color                     # => "red"
light.cycle!
light.color                     # => "green"