title: How to add a new column to an existing DataFrame?
body: <p>I have the following indexed DataFrame with named columns and rows not- continuous numbers:</p>
  
  <pre><code>          a         b         c         d
  2  0.671399  0.101208 -0.181532  0.241273
  3  0.446172 -0.243316  0.051767  1.577318
  5  0.614758  0.075793 -0.451460 -0.012493
  </code></pre>
  
  <p>I would like to add a new column, <code>'e'</code>, to the existing data frame and do not want to change anything in the data frame (i.e., the new column always has the same length as the DataFrame). </p>
  
  <pre><code>0   -0.335485
  1   -1.166658
  2   -0.385571
  dtype: float64
  </code></pre>
  
  <p>How can I add column <code>e</code> to the above example? </p>