title: R - how to re-order data frame by row index number
body: <p>This may be a very basic question but I could't find it. Let's say I have a data frame <code>d</code> with row numbers in disorder like this:</p>
  
  <pre><code>    Signal
  4   9998
  3   549
  1   18
  5   2.342
  2   0.043
  </code></pre>
  
  <p>How can I sort this by increasing row index numbers to obtain the following?</p>
  
  <pre><code>    Signal
  1   18
  2   0.043
  3   549
  4   9998
  5   2.342
  </code></pre>