title: how to put a column below another
body: <p>I would like to put the columns of a dataframe one below the other.</p>
  <p>I have this:</p>
  <pre><code>a &lt;- c(1, 2, 3, 4)
  b &lt;- c(5, 6, 7, 8)
  c &lt;- c(9, 10, 11, 12)
  abc &lt;- data.frame(a, b, c)
  </code></pre>
  <p>I would like to end up having this:</p>
  <pre><code>abc &lt;- c(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)
  </code></pre>
  <p>Note that in my original dataset I have multiple columns, so I would like to obtain a code without mentioning specific columns.</p>
  <p>Thanks!</p>