Compare files from different folders

Question

I would like to write a script that compares files from different folders; compare is done between different folder with the same filename. Eg

Folder A: filename1, filename2, filename3
Folder B: filename1, filename2, filename3

desired Output, written to an output file:

filename1:
different1
different2

filename2
differentA
differentB

Answer 1

Why not just use?

diff -ur path1 path2