title: Appending to numpy arrays
body: <p>I'm trying to construct a numpy array, and then append integers and another array to it.
  I tried doing this:</p>
  
  <pre><code>xyz_list = frag_str.split()
  nums = numpy.array([])
  coords = numpy.array([])
  for i in range(int(len(xyz_list)/4)):
      numpy.append(nums, xyz_list[i*4])
      numpy.append(coords, xyz_list[i*4+1:(i+1)*4])
  print(atoms)
  print(coords)
  </code></pre>
  
  <p>Printing out the output only gives my empty arrays. Why is that?
  In addition, how can I rewrite <code>coords</code> in a way that allows me to have 2D arrays like this: <code>array[[0,0,0],[0,0,1],[0,0,-1]]</code>?</p>