title: assign row name while rbind a row in a data frame
body: <p>I want to assign a rowname (A_B) while I rbind a row into a new dataframe (d). 
  The row is the result of the ratio of two rows of another data frame (df).</p>
  
  <pre><code>    df &lt;- data.frame(ID = c("A", "B" ),replicate(3,sample(1:100,2,rep=TRUE)))
      d &lt;- data.frame()
      d&lt;-rbind(d, df[df$ID == "A",2:4 ]/df[df$ID == "B", 2:4])
  </code></pre>
  
  <p>Actual output</p>
  
  <pre><code>    X1   X2   X3
  1 0.08 0.14 0.66
  </code></pre>
  
  <p>Expected output. Instead of rowname 1 I want A_B as result of A_B ratio</p>
  
  <pre><code>       X1   X2   X3
  A_B 0.08 0.14 0.66
  </code></pre>