title: Conditional calculating Maximum value in the column
body: <p>I have the following table:</p>
  
  <pre><code>Class  x2  x3  x4
    A    14  45  53 
    A     8  18  17
    A    16  49  20
    B    78  21  48 
    B     8  18   5
  </code></pre>
  
  <p>I need for each "Class" (A and B) find the maximum value in column "X3", keep that row and delete other rows.</p>
  
  <p>The output should be in format like:</p>
  
  <pre><code>Class  x2  x3  x4
    A    14  49  20
    B    78  21  48 
  </code></pre>
  
  <p>Please, ask me questions if something unclear in my problem.</p>
  
  <p>Thank you!</p>