title: Selecting max within partition for pandas dataframe
body: <p>I have a pandas dataframe. My goal is to select only those rows where column C has the largest value within group B. For example, when B is "one" the maximum value of C is 311, so I would like the row where C = 311 and B = "one."</p>
  
  <pre><code>import pandas as pd
  import numpy as np
  
  df2 = pd.DataFrame({ 'A' : 1., 
      'A' : pd.Categorical(["test1","test2","test3","test4"]),
      'B' : pd.Categorical(["one","one","two","two"]),
      'C' : np.array([311,42,31,41]),
      'D' : np.array([9,8,7,6])
      })
  
  df2.groupby('C').max()
  </code></pre>
  
  <p>Output should be:</p>
  
  <pre><code>test1 one 311 9
  test4 two 41  6
  </code></pre>