title: Select list of columns from a data frame using dplyr and select_()
body: <p>I'm trying to use the following function to extract some columns from a data frame:</p>
  
  <pre><code>library('dplyr')
  desired_columns = c(
    'a',
    'b',
    'c')
  extract_columns &lt;- function(data) {
    extracted_data &lt;- data %&gt;%
      select_(desired_columns)
    return(extracted_data)
  }
  </code></pre>
  
  <p>But when I try it, I don't get what I expect:</p>
  
  <pre><code>&gt; df &lt;- data.frame(a=1:5, b=1:5, c=1:5, d=1:5)
  &gt; df
    a b c d
  1 1 1 1 1
  2 2 2 2 2
  3 3 3 3 3
  4 4 4 4 4
  5 5 5 5 5
  &gt; extract_columns(df)
    a
  1 1
  2 2
  3 3
  4 4
  5 5
  </code></pre>
  
  <p>I seem to be only getting the first column and I can't figure out what I'm doing wrong. How can I get all the requested columns?</p>