title: Select row with most recent date by group
body: <p>I have a data frame in R where the rows represent events, and one column is the date of the event. The thing the event is happening to is described by an ID column. So for each ID there are multiple entries.</p>
  
  <p>How do I filter the data frame so that I retain only the most recent event for each ID? The IDs are integers and the dates are in the form <code>mm/dd/yyyy</code>.  </p>