With neural networks, we are working with sets of matrices:

In order to use optimizing functions such as "fminunc()", we will want to "unroll" all the elements and put them into one long vector:

If the dimensions of Theta1 is 10x11, Theta2 is 10x11 and Theta3 is 1x11, then we can get back our original matrices from the "unrolled" versions as follows:

To summarize:

来源：coursera 斯坦福 吴恩达 机器学习