title: Refactor whole data frame
body: <p>I have a data frame that I have read from disk and then applied a filter:</p>
  
  <pre><code>df &lt;- df[ df$x &gt; 10, ]
  </code></pre>
  
  <p><strong>Question:</strong> How can I refactor all factors in the data frame now that several rows have been removed?</p>