title: Finding the indices of the rows where there are non-zero entries in a sparse csc_matrix
body: <p>I have a numpy array, X:</p>
  
  <pre><code>type(X)
  &gt;&gt;&gt; &lt;class 'scipy.sparse.csc.csc_matrix'&gt;
  </code></pre>
  
  <p>I am interested in finding the indices of the rows where there are non-zero entries, in the 0th column.  I tried:</p>
  
  <pre><code>getcol =  X.getcol(0)
  print getcol
  </code></pre>
  
  <p>which gives me:</p>
  
  <pre><code>(0, 0)  1
  (2, 0)  1
  (5, 0)  10
  </code></pre>
  
  <p>This is great, but what I want is a vector that has <code>0, 2, 5</code> in it.  </p>
  
  <p>How do I get the indices I'm looking for?</p>
  
  <p>Thanks for the help.</p>