title: Numpy: find index of the elements within range
body: <p>I have a numpy array of numbers, for example,  </p>
  
  <pre><code>a = np.array([1, 3, 5, 6, 9, 10, 14, 15, 56])  
  </code></pre>
  
  <p>I would like to find all the indexes of the elements within a specific range. For instance, if the range is (6, 10), the answer should be (3, 4, 5). Is there a built-in function to do this?</p>