title: transform all continuous variables in a data.table into quantiles
body: <p>question is straight forward, how do I transform all the continuous variables in a data.table into quantiles of those variables.</p>
  <p>so for example</p>
  <pre><code>a   b  c  id
  1   4  6  123
  43  2  34 124
  </code></pre>
  <p>Should look like</p>
  <pre><code>a   b   c   id
  q1  q2  q1  123
  q2  q1  q2  124
  </code></pre>