Week 3 - assigment 2 - question on selection of specific elements for contracting and expanding blocks


I’ve finished the assignment successfully but I’m not sure I understand why in the contracting block we have to “Chain the first element of the output of each block”

and in the expanding we have to chain the second element.

Can someone explain to me the rationale here?


I won’t go too far, so I would say, it is because we need those two elements to make the two kinds of connection as illustrated below.

and because in the conv_block function, we have arranged the two elements in this order:


The lecture video " U-Net Architecture Intuition" in Course 4 Week 3 has discussed the contraction and skip connection. You might want to review them if something is not clear.