title: Find the max date in a single column across multiple rows
body: <p>I have the following data frame:</p>
  
  <pre><code>id       &lt;- c(1,1,2,3,3)
  date     &lt;- c("23-01-08","01-11-07","30-11-07","17-12-07","12-12-08")
  df       &lt;- data.frame(id,date)
  df$date2 &lt;- as.Date(as.character(df$date), format = "%d-%m-%y")
  
  
  id     date      date2
  1   23-01-08 2008-01-23
  1   01-11-07 2007-11-01
  2   30-11-07 2007-11-30
  3   17-12-07 2007-12-17
  3   12-12-08 2008-12-12
  </code></pre>
  
  <p>now I need to create a forth column and insert maximum date of transaction for each <code>id</code> in that.
  the final table should be as follow:</p>
  
  <pre><code>id     date      date2        max
  1   23-01-08 2008-01-23 2008-01-23
  1   01-11-07 2007-11-01   0
  2   30-11-07 2007-11-30 2007-11-30 
  3   17-12-07 2007-12-17   0
  3   12-12-08 2008-12-12 2008-12-12
  </code></pre>
  
  <p>I would be thankful if you could help me with this.</p>