title: Initialise numpy array of unknown length
body: <p>I want to be able to 'build' a numpy array on the fly, I do not know the size of this array in advance.</p>
  
  <p>For example I want to do something like this:</p>
  
  <pre><code>a= np.array()
  for x in y:
       a.append(x)
  </code></pre>
  
  <p>Which would result in a containing all the elements of x, obviously this is a trivial answer. I am just curious whether this is possible?</p>