title: How to get indices of a 2D array from index of the masked array
body: <p>I need the original indices of a 2D array in a loop over the index of a masked array. I try:</p>
  
  <pre><code>for i in range(Arr[mask].size):
      tmp = numpy.zeros(mask.shape, dtype=bool)
      tmp[mask][i] = True
      j,k = numpy.where(tmp)
      ...
  </code></pre>
  
  <p>Unfortunately, last line returns</p>
  
  <pre><code>(array([], dtype=int64), array([], dtype=int64))   
  </code></pre>
  
  <p>What is a good way to find j and k?</p>