028_number_spiral_diagonals.c

/*
Starting with the number 1 and moving to the right in a clockwise direction a 5 by 5 spiral is
formed as follows:

21 22 23 24 25
20  7  8  9 10
19  6  1  2 11
18  5  4  3 12
17 16 15 14 13

It can be verified that the sum of the numbers on the diagonals is 101.

What is the sum of the numbers on the diagonals in a 1001 by 1001 spiral formed in the same way?
*/

#include <stdio.h>

int main(int argc, const char *argv[])
{
    int result = 1;
    int tmp = 1;
    int incrementer = 2;

    for(int i = 0; i < 500; i++){
        for(int j = 0; j < 4; j++){
            tmp += incrementer;
            result += tmp;
        }
        incrementer += 2;
    }
    printf("result= %d\n", result);
    return 0;
}