title: arrange() not working on grouped data frame
body: <p>Assume I have the following code. In the last step, where I try arranging it, the code doesn't work and the data frame continues to be arranged in ascending order by <code>cyl</code>.</p>
  
  <pre><code>library(dplyr)
  # create a grouped data frame
  df &lt;- group_by(mtcars,cyl)
  # rank car from best mpg to worst for every cyl
  df &lt;- mutate(df,rank = row_number(mpg)) 
  # top 3 best mpg for each cyl
  df &lt;- filter(df,rank &lt;= 3) 
  # arrange by the number of cyl
  df &lt;- arrange(df,desc(cyl), rank) 
  </code></pre>
  
  <p>Any thoughts on why this is happening ? </p>