title: The meaning of the comma inside X[:,0]
body: <p>If <code>X</code> is an array, what is the meaning of <code>X[:,0]</code>? In fact, it is not the first time I see such thing, and it's confusing me, but I can't see what is its meaning? Could anyone be able to show me an example? A full clear answer would be appreciated on this question of comma.</p>
  <p>Please see the file <a href="https://github.com/lazyprogrammer/machine_learning_examples/blob/master/ann_class/forwardprop.py" rel="nofollow noreferrer">https://github.com/lazyprogrammer/machine_learning_examples/blob/master/ann_class/forwardprop.py</a></p>