title: Filter data frame to keep only maximum value from each group
body: <pre><code>a &lt;- c(rep("A", 3), rep("B", 3), rep("C",2))
  b &lt;- c(1,1,2,4,1,1,2,2)
  df &lt;- data.frame(a,b)
  </code></pre>
  
  <p>But I want to keep the highest <code>b</code> value of <code>"A"</code>, <code>"B"</code>, and <code>"C"</code>.</p>
  
  <p>So my new df only has 3 rows. Thank you!!!</p>