title: Updating a NumPy array with another
body: <p>Seemingly simple question: I have an array with two columns, the first represents an ID and the second a count.  I'd like to update it with another, similar array such that</p>
  
  <pre><code>import numpy as np
  
  a = np.array([[1, 2],
                [2, 2],
                [3, 1],
                [4, 5]])
  
  b = np.array([[2, 2],
                [3, 1],
                [4, 0],
                [5, 3]])
  
  a.update(b)  # ????
  &gt;&gt;&gt; np.array([[1, 2],
                [2, 4],
                [3, 2],
                [4, 5],
                [5, 3]])
  </code></pre>
  
  <p>Is there a way to do this with indexing/slicing such that I don't simply have to iterate over each row?</p>