title: slicing sparse (scipy) matrix
body: <p>I would appreciate any help, to understand following behavior when slicing a lil_matrix (A) from the scipy.sparse package.</p>
  
  <p>Actually, I would like to extract a submatrix based on an arbitrary index list for both rows and columns.</p>
  
  <p>When I used this two lines of code:</p>
  
  <pre><code>x1 = A[list 1,:]
  x2 = x1[:,list 2]
  </code></pre>
  
  <p>Everything was fine and I could extract the right submatrix.</p>
  
  <p>When I tried to do this in one line, it failed (The returning matrix was empty)</p>
  
  <pre><code>x=A[list 1,list 2]
  </code></pre>
  
  <p>Why is this so? Overall, I have used a similar command in matlab and there it works.
  So, why not use the first, since it works? It seems to be quite time consuming. Since I have to go through a large amount of entries, I would like to speed it up using a single command. Maybe I use the wrong sparse matrix type...Any idea?</p>