A simple code to initialize a matrix of N*N elements with all same number can be to create 2 for loops from 0 to N and set element to the required number. A silght improvement can be done to this code by
for (int i = 0; i < N / 2; i++) for (int j = i; j < N - i; j++) { arr[i][j] = 0; arr[j][i] = 0; arr[N - 1 - i][N - 1 - j] = 0; arr[N - 1 - j][N - 1 - i] = 0; }