original_with_magic_numbers.rb

# Original module with "magic numbers" for 4 suits and 13 card values
# See: http://refactoring.com/catalog/replaceMagicNumberWithSymbolicConstant.html

module PlayingCards
  SUITS = %w{ spades clubs hearts diamonds }

  class Deck
    attr_accessor :cards

    def initialize
      @cards = []
      4.times do |s|
        13.times do |r|
          @cards << Card.new(r, SUITS[s])
        end
      end
    end
  end

  class Card
    def initialize(rank, suit)
      @rank, @suit = rank, suit
    end
  end
end

refactored_with_constants.rb

module PlayingCards
  SUITS = %w{ spades clubs hearts diamonds }
  RANKS = %w{ 2 3 4 5 6 7 8 9 10 J Q K A }

  class Deck
    attr_accessor :cards

    def initialize
      @cards = []
      SUITS.each do |suit|
        RANKS.each do |rank|
          @cards << Card.new(rank, suit)
        end
      end
    end
  end

  class Card
    def initialize(rank, suit)
      @rank, @suit = rank, suit
    end
  end
end