title: How to add empty rows to a data frame?
body: <p>I am creating data.frames with by_group.
  Now the number of rows depends on the dataset. Some have more rows than others. 
  But i always need the same number.
  Is there a more efficient way for doubling the dataset by adding the same number of rows, but with empty values?</p>
  
  <pre><code>#add empty rows to dataframe
  
  library(dplyr)
  
  var1 &lt;- 1:10
  var2 &lt;- c(1,1,1,1,0,1,0,1,1,0)
  var3 &lt;- rep(3, 10)
  
  dat &lt;- data.frame (var1, var2, var3)
  View(dat)
  
  
  dat2 &lt;- data.frame (var1=rep(NA, 10), var2=rep(NA, 10), var3=rep(NA, 10))
  bind_rows(dat, dat2)
  </code></pre>