title: Indices of multiple elements in a numpy array
body: <p>I have a numpy array and a list as follows</p>
  
  <pre><code>y=np.array([[1],[2],[1],[3],[1],[3],[2],[2]])
  x=[1,2,3]
  </code></pre>
  
  <p>I would like to return a tuple of arrays each of which contains the indices of each element of x in y.
  i.e.</p>
  
  <pre><code>(array([[0,2,4]]),array([[1,6,7]]),array([[3,5]]))
  </code></pre>
  
  <p>Is this possible to be done in a vectorized fashion(without any loops)?</p>