Given an M × N matrix mat that has all of its elements initialized to 0, write a C++ code fragment to store 1's along mat's main diagonal while leaving all other entries unchanged.

Example:
1 0 0 0
0 1 0 0
0 0 1 0

for  (int i = 0;  i < M;  ++i)

   mat[i][i] = 1;