title: Apply function to result of numpy.where
body: <p>Assume I have an array:</p>
  
  <pre><code>a = [1,2,3,4,0,-1,-2,3,4]
  </code></pre>
  
  <p>Using <code>np.where(a &lt; 0)</code> it returns a list of indices which elements a are &lt; 0 </p>
  
  <p>How to apply a function for those elements of a? </p>