title: Add one column below another in a data.frame in R
body: <p>Not sure if I used the correct english words in the subject to describe what I need.</p>
  
  <p>See this example</p>
  
  <pre><code>df &lt;- data.frame(a=sample(1:10), b=sample(1:10))
  df
      a  b
  1   2  9
  2   5  8
  3  10  1
  4   3  7
  5   8  4
  6   6 10
  7   9  5
  8   7  6
  9   1  3
  10  4  2
  </code></pre>
  
  <p>I want to take <code>b</code> column at put it below <code>a</code> as 10 new rows.</p>
  
  <pre><code>    a
  1   2
  2   5
  3  10
  4   3
  5   8
  6   6
  7   9
  8   7
  9   1
  10  4
  11  9
  12  8
  13  1
  14  7
  15  4
  16 10
  17  5
  18  6
  19  3
  20  2
  </code></pre>
  
  <p>I found examples with <code>rbind</code> but couldn't find out how to use it in my situation.</p>