title: R dplyr get sorted row number
body: <p>dplyr is returning the original row number and not the current row number: </p>
  
  <pre><code>df &lt;- data.frame(a=c("A", "B"), b=c(1,2))
  ----------------
  a   b
  A   1
  B   2
  -----------------
  df %&gt;% arrange(desc(b)) %&gt;% mutate(row_number(a))
  -----------------
  a   b   row_number(a)
  B   2   2
  A   1   1
  </code></pre>
  
  <p>how do I get a column that returns 1,2?</p>