title: Find index of multiple elements in numpy array
body: <p>Given an array of integers, I need to find the indexes of many of its elements stored in a different array. This is:</p>
  
  <pre><code>import numpy as np
  a1 = np.array([ 4, 5, 6, 1, 2, 3, 7, 86, 9, 15])
  a2 = np.array([ 2, 3, 5, 6, 9])
  </code></pre>
  
  <p>Where <code>a1</code> is my initial array of elements, and <code>a2</code> is the array that contains the elements for which I need their indexes in <code>a1</code>.</p>
  
  <p>In this case, the result should be:</p>
  
  <pre><code>a3 = ([4, 5, 1, 2, 8])
  </code></pre>
  
  <p>This seems like a rather simple operation, but I haven't been able to figure out how to do it.</p>