title: levels function alternative for data table R
body: <p>If I'm not mistaken, the levels function doesn't apply to a data table (correct me if wrong, i'm new to R)</p>
  
  <pre><code>&gt; levels(DT$C1)
  NULL
  </code></pre>
  
  <p>Is there an alternative for a data table? I'm not finding anything usefull. </p>