title: What does += mean in Ruby?
body: <blockquote>
    <p><strong>Possible Duplicate:</strong><br>
    <a href="https://stackoverflow.com/questions/7638502/what-does-plus-equals-mean">What does “+=” (plus equals) mean?</a>  </p>
  </blockquote>
  
  
  
  <p>What is the point of <code>sum += square</code> below? What does it mean?</p>
  
  <pre><code>sum = 0
  [1, 2, 3, 4].each do |value|
    square = value * value
    sum += square
  end
  puts sum
  </code></pre>