title: how to remove columns from data frame in R
body: <p>I am trying to remove columns from data.frame. 
  The following expression works   </p>
  
  <pre><code>a1$"STR_COL_4_" &lt;- NULL 
  </code></pre>
  
  <p>but   </p>
  
  <pre><code>xx &lt;- "STR_COL_4_"   
  a1$xx &lt;- NULL    
  </code></pre>
  
  <p>does not work...what is the problem ???</p>