title: Error when unlisting columns in a data frame
body: <p>Suppose I have a data frame called DF:</p>
  
  <pre><code>options(stringsAsFactors = F)
  
  letters &lt;- list("A", "B", "C", "D")
  numbers &lt;- list(list(1,2), 1, 1, 2)
  score &lt;- list(.44, .54, .21, .102)
  
  DF &lt;- data.frame(cbind(letters, numbers, score))
  </code></pre>
  
  <p>Note that all columns in the data frame are of class "list".</p>
  
  <p>Also, take a look at the structure: DF$numbers[1] is also a list </p>
  
  <p>I'm trying to UNLIST each column.</p>
  
  <pre><code> DF$letters &lt;- unlist(DF$letters)
   DF$score &lt;- unlist(DF$score)
   DF$numbers &lt;- unlist(DF$numbers)
  </code></pre>
  
  <p>However, because, DF$numbers[1] is also a list, I'm thrown back this error:</p>
  
  <pre><code> Error in `$&lt;-.data.frame`(`*tmp*`, numbers, value = c(1, 2, 1, 1, 2)) : 
     replacement has 5 rows, data has 4
  </code></pre>
  
  <p>Is there a way that I can unlist the whole column, and keep the values cells like DF$numbers[1] as a character vector like c(1,2) or 1,2?</p>
  
  <p>Ideally I would like DF to look something like this, where the individual values in the <strong>number column are still of type int:</strong></p>
  
  <pre><code> letters   numbers   score
    A         1,2       .44
    B         1         .54
    C         1         .21
    D         2         .102
  </code></pre>
  
  <p>The goal is to then write the data frame to a csv file.</p>