title: Numpy: return boolean array after condition checking
body: <p>Simple question: I know that stuff like</p>
  
  <pre><code>import numpy as np
  
  a = np.array([1, 2])
  (a &lt; 2)
  </code></pre>
  
  <p>will return a boolean array where the condition (elem &lt; 2) was checked:</p>
  
  <pre><code>array([True, False], dtype=bool)
  </code></pre>
  
  <p>Is this a native numpy method? I searched for the documentation of this but were not able to find any :(</p>
  
  <p>Can someone supply a link for the documentation?</p>
  
  <p>Is something like <code>(a &lt; 2).sum()</code> considered "good style" for counting the amount of elements which fulfill a certain condition?</p>
  
  <p>...and finally: are there faster ways for turning an array to a boolean array based on a condition? Or is this already an efficient way for checking each element against a condition?</p>