title: How to group rows in a Numpy 2D matrix based on column values?
body: <p>What  would be an efficient (time, easy) way of grouping a <code>2D</code> <code>NumPy</code> matrix rows by different column conditions (e.g. group by column 2 values) and running <code>f1()</code> and <code>f2()</code> on each of those groups?</p>
  
  <p>Thanks</p>