title: Find rows in dataframe with maximum values grouped by values in another column
body: <p>I would like to solve this problem in R without using SQL. </p>
  
  <p><a href="https://stackoverflow.com/questions/612231/how-can-i-select-rows-with-maxcolumn-value-distinct-by-another-column-in-sql">How can I SELECT rows with MAX(Column value), DISTINCT by another column in SQL?</a></p>
  
  <p>Sure, I could use sqldf to do it, but there must be a cool apply method in R to do it, too?</p>