title: selecting certain rows in a data frame
body: <p>I like to retrieve rows that meet some criteria. for example, from the below set up numbers, I like to retrieve rows Percent_free &lt; 10. How would I do this in R?</p>
  
  <pre><code> Date             file                 Percent_free
  2011-12-06            /tmp             2
  2011-12-06            /var             9
  2011-12-06            /crash           11
  </code></pre>