title: Numpy array Conditional Updating
body: <p>Let's say I have a numpy array.</p>
  <pre><code>array([[1, 1, 1],
         [5, 1, 4]])
  </code></pre>
  <p>Now I want to update all rows whose first value is 1 to 4.</p>
  <p>My code is</p>
  <pre><code>arr[arr[:,0]==1][:, 0]=4
  </code></pre>
  <p>But when I see array. It is not updated</p>
  <pre><code>In [80]: arr
  Out[80]: 
  array([[1, 1, 1],
         [5, 1, 4]])
  
  </code></pre>