title: How to mask one ndarray using a another ndarray
body: <p>I have an <code>items</code> array which has shape <code>(n, 3)</code>, and a <code>counts</code> array which has the same shape <code>(n, 3)</code>. How can I make every <code>C</code> in <code>items</code> to have <code>count = 0</code> without resorting to loops?</p>
  
  <pre><code>items = np.array([['B', 'A', 'C'],
      ['B', 'B', 'C'],
      ['A', 'B', 'A'],
      ['C', 'C', 'C'],
      ['B', 'B', 'B']])
  
  counts = np.array([[1, 3, 2],
      [4, 2, 3],
      [2, 2, 1],
      [3, 2, 1],
      [1, 2, 1]])
  </code></pre>
  
  <p>Expected output:</p>
  
  <pre><code>&gt;&gt;&gt; counts
  np.array([[1, 3, 0],
      [4, 2, 0],
      [2, 2, 1],
      [0, 0, 0],
      [1, 2, 1]])
  </code></pre>