title: Convert a list of dictionaries to numpy matrix?
body: <p>Let the given dictionaries are </p>
  
  <pre><code>d = [{'a':1,'b':4},{'b':2}]
  </code></pre>
  
  <p>So basically I want a matrix like this</p>
  
  <pre><code>  | 'a' | 'b'  |
   _______________
    |  1  |  4   |
    |  na |  2   |
  </code></pre>
  
  <p>How can I efficiently achieve this ? </p>