title: Replace negative values in an numpy array
body: <p>Is there a simple way of replacing all negative values in an array with 0?</p>
  <p>I'm having a complete block on how to do it using a NumPy array.</p>
  <p>E.g.</p>
  <pre><code>a = array([1, 2, 3, -4, 5])
  </code></pre>
  <p>I need to return</p>
  <pre><code>[1, 2, 3, 0, 5]
  </code></pre>
  <p><code>a &lt; 0</code> gives:</p>
  <pre><code>[False, False, False, True, False]
  </code></pre>
  <p>This is where I'm stuck - how to use this array to modify the original array.</p>