title: add a string prefix to each value in a string column using Pandas
body: <p>I would like to append a string to the start of each value in a said column of a pandas dataframe (elegantly).
  I already figured out how to kind-of do this and I am currently using:</p>
  
  <pre><code>df.ix[(df['col'] != False), 'col'] = 'str'+df[(df['col'] != False), 'col']
  </code></pre>
  
  <p>This seems one hell of an inelegant thing to do - do you know any other way (which maybe also adds the character to rows where that column is 0 or NaN)?</p>
  
  <p>In case this is yet unclear, I would like to turn:</p>
  
  <pre><code>    col 
  1     a
  2     0
  </code></pre>
  
  <p>into:</p>
  
  <pre><code>       col 
  1     stra
  2     str0
  </code></pre>