title: Getting an error in my Ruby script
body: <pre><code>class Card
  
    attr_accessor :number, :suit
  
    def initialize(number, suit)
      @number = number
      @suit = suit
    end
  
    def to_s
      "#{@number} of #{@suit}"
    end
  end
  </code></pre>
  
  <p>I'm assuming this creates a new array correct?
  But why the use of the AT symbol? When should I use it and not use it?</p>
  
  <pre><code>@stack_of_cards = []
  
  @stack &lt;&lt; Card.new("A", "Spades")
  
  puts @stack
  
  # =&gt; BlackjackGame.rb:17: undefined method `&lt;&lt;' for nil:NilClass (NoMethodError)
  </code></pre>
  
  <p>Any ideas why this error is firing?</p>