title: find the set of column indices for non-zero values in each row in pandas' data frame
body: <p>Is there a good way to find the set of column indices for non-zero values in each row in pandas' data frame? Do I have to traverse the data frame row-by-row?</p>
  
  <p>For example, the data frame is</p>
  
  <pre><code>c1  c2  c3  c4 c5 c6 c7 c8  c9
   1   1   0   0  0  0  0  0   0
   1   0   0   0  0  0  0  0   0
   0   1   0   0  0  0  0  0   0
   1   0   0   0  0  0  0  0   0
   0   1   0   0  0  0  0  0   0
   0   0   0   0  0  0  0  0   0
   0   2   1   1  1  1  1  0   2
   1   5   5   0  0  1  0  4   6
   4   3   0   1  1  1  1  5  10
   3   5   2   4  1  2  2  1   3
   6   4   0   1  0  0  0  0   0
   3   9   1   0  1  0  2  1   0
  </code></pre>
  
  <p>The output is expected to be</p>
  
  <pre><code>['c1','c2']
  ['c1']
  ['c2']
  ...
  </code></pre>