title: Accessing Lower Triangle of a Numpy Matrix?
body: <p>Okay, so basically lets say i have a matrix:</p>
  
  <pre><code>matrix([[0, 1, 2, 3, 4],
          [0, 1, 2, 3, 4],
          [0, 1, 2, 3, 4],
          [0, 1, 2, 3, 4],
          [0, 1, 2, 3, 4]])
  </code></pre>
  
  <p>Is it possible to get the area below the diagonal easily when working with numpy matrixs? I looked around and could not find anything. I can do the standard, for loop way however wouldnt that somehow invalidate the performance gave by numpy?</p>
  
  <p>I am working on calculating statististics ofcomparing model output results with actual results. The data i currently have been given, results in around a 10,000 x 10,000 matrix. I am mainly trying to just sum those elements. </p>
  
  <p>Is there an easy way to do this?</p>