title: numpy get index where value is true
body: <pre><code>&gt;&gt;&gt; ex=np.arange(30)
  &gt;&gt;&gt; e=np.reshape(ex,[3,10])
  &gt;&gt;&gt; e
  array([[ 0,  1,  2,  3,  4,  5,  6,  7,  8,  9],
         [10, 11, 12, 13, 14, 15, 16, 17, 18, 19],
         [20, 21, 22, 23, 24, 25, 26, 27, 28, 29]])
  &gt;&gt;&gt; e&gt;15
  array([[False, False, False, False, False, False, False, False, False,
          False],
         [False, False, False, False, False, False,  True,  True,  True,
           True],
         [ True,  True,  True,  True,  True,  True,  True,  True,  True,
           True]], dtype=bool)
  </code></pre>
  
  <p>I need to find the rows that have true or rows in <code>e</code> whose value are more than 15. I could iterate using a for loop, however, I would like to know if there is a way numpy could do this more efficiently?</p>