title: how to drop columns by passing variable name with dplyr?
body: <p>I have a df as follows:</p>
  <pre><code>a &lt;- data_frame(keep=c(&quot;hello&quot;, &quot;world&quot;),drop = c(&quot;nice&quot;, &quot;work&quot;))
  a
  Source: local data frame [2 x 2]
     keep  drop
    (chr) (chr)
  1 hello  nice
  2 world  work
  </code></pre>
  <p>I can use <code>a  %&gt;% select(-drop) </code> to drop the column without problem. however, if I want to pass a variable to present <code>drop</code> column, then it returns error.</p>
  <pre><code>name &lt;- &quot;drop&quot;
  a  %&gt;% select(-(name))
  </code></pre>
  <blockquote>
  <p>Error in -(name) : invalid argument to unary operator</p>
  </blockquote>