title: Split and unsplit a dataframe in four parts
body: <p>I'd like to split a dataframe in 4 equals parts, because I'd like to use the 4 cores of my computer.</p>
  
  <p>I did this :</p>
  
  <pre><code>df2 &lt;- split(df, 1:4)
  unsplit(df2, f=1:4)
  </code></pre>
  
  <p>and that</p>
  
  <pre><code>df2 &lt;- split(df, 1:4)
  unsplit(df2, f=c('1','2','3','4')
  </code></pre>
  
  <p>But the unsplit function did not work, I have these warnings messages</p>
  
  <pre><code>1: In split.default(seq_along(x), f, drop = drop, ...) :
    data length is not a multiple of split variable
  ...
  </code></pre>
  
  <p>Do you have an idea of the reason ?</p>