title: Getting the integer index of a Pandas DataFrame row fulfilling a condition?
body: <p>I have the following DataFrame:</p>
  
  <pre><code>   a  b  c
  b
  2  1  2  3
  5  4  5  6
  </code></pre>
  
  <p>As you can see, column <code>b</code> is used as an index. I want to get the ordinal number of the row fulfilling <code>('b' == 5)</code>, which in this case would be <code>1</code>.</p>
  
  <p>The column being tested can be either an index column (as with <code>b</code> in this case) or a regular column, e.g. I may want to find the index of the row fulfilling <code>('c' == 6)</code>.</p>