title: Select the row with the maximum value in each group
body: <p>In a dataset with multiple observations for each subject. For each subject I want to select the row which have the maximum value of 'pt'. For example, with a following dataset:</p>
  <pre><code>ID    &lt;- c(1,1,1,2,2,2,2,3,3)
  Value &lt;- c(2,3,5,2,5,8,17,3,5)
  Event &lt;- c(1,1,2,1,2,1,2,2,2)
  
  group &lt;- data.frame(Subject=ID, pt=Value, Event=Event)
  #   Subject pt Event
  # 1       1  2     1
  # 2       1  3     1
  # 3       1  5     2 # max 'pt' for Subject 1
  # 4       2  2     1
  # 5       2  5     2
  # 6       2  8     1
  # 7       2 17     2 # max 'pt' for Subject 2
  # 8       3  3     2
  # 9       3  5     2 # max 'pt' for Subject 3
  </code></pre>
  <p>Subject 1, 2, and 3 have the biggest pt value of 5, 17, and 5 respectively.</p>
  <p>How could I first find the biggest pt value for each subject, and then, put this observation in another data frame? The resulting data frame should only have the biggest pt values for each subject.</p>