title: Calculate percentile for every value in a column of dataframe
body: <p>I am trying to calculate percentile for every value in column <code>a</code> from a DataFrame <code>x</code>.</p>
  
  <p>Is there a better way to write the following piece of code?</p>
  
  <pre><code>x["pcta"] = [stats.percentileofscore(x["a"].values, i) 
                                      for i in x["a"].values]
  </code></pre>
  
  <p>I would like to see better performance.</p>