title: Pandas (python): How to add column to dataframe for index?
body: <p>The index that I have in the dataframe (with 30 rows) is of the form:</p>
  <pre><code>Int64Index([171, 174,173, 172, 199..............
          ....175, 200])
  </code></pre>
  <p>The index is not strictly increasing because the data frame is the output of a sort().
  I want to have add a column which is the series:</p>
  <pre><code>[1, 2, 3, 4, 5......................., 30]
  </code></pre>
  <p>How should I go about doing that?</p>