# [✓] Add plus 1 to the {i_,i_} elements in a list?

GROUPS:
 I have a list of lists of numbers, for example: {{1,1,1,1}, {1,1,1,1},{1,1,1,1},{1,1,1,1}} The length of the list is the same length of each sublist. What I want to achieve is to add plus 1 to the {i,i} elements without using For cycle. Given the list above as an input, I need a result like this: {{2,1,1,1}, {1,2,1,1},{1,1,2,1},{1,1,1,2}} 
 Szabolcs Horvát 7 Votes It sounds like you have a square matrix, and you want to add IdentityMatrix@Length[matrix] to it.