title: Access an instance variable without using the @ sign
body: <p>Here is an example from a book:</p>
  
  <pre><code>class TextCompressor
    attr_reader :unique, :index
    def initialize( text )
      @unique = []
      @index = []
      add_text( text )
    end
    def add_text( text )
      words = text.split
      words.each { |word| add_word( word ) }
    end
    def add_word( word )
      i = unique_index_of( word ) || add_unique_word( word )
      @index &lt;&lt; i
    end
    def unique_index_of( word )
      @unique.index(word)
    end
    def add_unique_word( word )
      @unique &lt;&lt; word
      unique.size - 1
    end
  end
  </code></pre>
  
  <p>In the method <code>add_unique_word</code> the author access the variable <code>unique</code> without using the @ sign (<code>unique.size - 1</code>). How is it possible, and why it is so?</p>