title: Create matrix in a loop with numpy
body: <p>I would like to build up a numpy matrix using rows I get in a loop. But how do I initialize the matrix? If I write</p>
  
  <pre><code>A = []
  A = numpy.vstack((A, [1, 2]))
  </code></pre>
  
  <p>I get</p>
  
  <p>ValueError: all the input array dimensions except for the concatenation axis must match exactly</p>
  
  <p>What's the best practice for this?</p>
  
  <p>NOTE: I do not know the number of rows in advance. The number of columns is known.</p>