title: Fast replacement of values in a numpy array
body: <p>I have a very large numpy array (containing up to a million elements) like the one below:</p>
  <pre><code>[0,1,6,5,1,2,7,6,2,3,8,7,3,4,9,8,5,6,11,10,6,7,12,11,7,
  8,13,12,8,9,14,13,10,11,16,15,11,12,17,16,12,13,18,17,13,
  14,19,18,15,16,21,20,16,17,22,21,17,18,23,22,18,19,24,23]
  </code></pre>
  <p>and a small dictionary map for replacing some of the elements in the above array</p>
  <pre><code>{4: 0, 9: 5, 14: 10, 19: 15, 20: 0, 21: 1, 22: 2, 23: 3, 24: 0}
  </code></pre>
  <p>I would like to replace some of the elements according to the map above. The numpy array is really large, and only a small subset of the elements (occurring as keys in the dictionary) will be replaced with the corresponding values. What is the fastest way to do this?</p>