title: Numpy tutorial - Boolean indexing
body: <p>Reading Numpy quick tutorial, I cannot understand this sentence.</p>
  
  <pre><code>a = np.arange(12).reshape(3,4)
  b1 = np.array([False,True,True]) 
  b2 = np.array([True,False,True,False])
  &gt;&gt;&gt; a[b1,b2]  
  array([ 4, 10])
  </code></pre>
  
  <p>Why <code>a[b1,b2]</code> is <code>array([4,10])</code> instead of <code>array([[4,6],[8,10]])</code>?</p>