title: Split data.frame by value
body: <p>how can I split the following data.frame</p>
  
  <pre><code>df &lt;- data.frame(var1 = c("a", 1, 2, 3, "a", 1, 2, 3, 4, 5, 6, "a", 1, 2), var2 = 1:14)
  </code></pre>
  
  <p>into lists of / groups of</p>
  
  <pre><code>a 1
  1 2
  2 3
  3 4
  
  a 5
  1 6
  2 7
  3 8
  4 9
  5 10
  6 11
  
  a 12
  1 13
  2 14
  </code></pre>
  
  <p>So basically, value "a" in column 1 is the tag / identifier I want to split the data frame on. I know about the split function but that means I have to add another column and since, as can be seen from my example, the size of the groups can vary I do not know how to automatically create such a dummy column to fit my needs.</p>
  
  <p>Any ideas on that?</p>
  
  <p>Cheers,</p>
  
  <p>Sven</p>