One of the other reasons I have you loop through everything is to avoid cut-and-pastes from this site http://aimotion.blogspot.com/2011/10/machine-learning-with-python-linear.html. You can't just use his code without modifying it significantly if you want to avoid vectorization. I also want you to see the speed difference when vectorization is used. Use code on the internet when implementing core functions only if you change it significantly.