title: remove zero lines 2-D numpy array
body: <p>I run a <code>qr factorization</code> in <code>numpy</code> which returns a list of <code>ndarrays</code>, namely <code>Q</code>and <code>R</code>:</p>
  
  <pre><code>&gt;&gt;&gt; [q,r] = np.linalg.qr(np.array([1,0,0,0,1,1,1,1,1]).reshape(3,3))
  </code></pre>
  
  <p><code>R</code> is a two-dimensional array, having pivoted zero-lines at the bottom (even proved for all examples in my test set):</p>
  
  <pre><code>&gt;&gt;&gt; print r
  [[ 1.41421356  0.70710678  0.70710678]
   [ 0.          1.22474487  1.22474487]
   [ 0.          0.          0.        ]]
  </code></pre>
  
  <p>. Now, I want to divide <code>R</code> in two matrices <code>R_~</code>:</p>
  
  <pre><code>[[ 1.41421356  0.70710678  0.70710678]
   [ 0.          1.22474487  1.22474487]]
  </code></pre>
  
  <p>and <code>R_0</code>:</p>
  
  <pre><code>[[ 0.          0.          0.        ]]
  </code></pre>
  
  <p>(extracting all zero-lines). It seems to be close to this solution: <a href="https://stackoverflow.com/questions/3877491/deleting-rows-in-numpy-array">deleting rows in numpy array</a>.</p>
  
  <p>EDIT:<br>
  Even more interesting: <code>np.linalg.qr()</code> returns a <code>n x n</code>-matrix. Not, what I would have expected:</p>
  
  <pre><code>A := n x m
  Q := n x m
  R := n x m
  </code></pre>