solution of array addition problem

#include <stdio.h>

int i, j, n, sum;
int diagsum_l2r[15]; //diagonal sums from left-to-right go here
int diagsum_r2l[15]; //diagonal sums from right to left go here

//this is the code for the "\" (left-to-right) diagonals
    for (n = 0; n <= 7; n++) //n counts the squares of each diagonal
    {
        sum = 0; //initialize sum to 0, here goes the sum of each diagonal line
        for (i = 0; i <= n; i++)
        {
            for (j = 0; j <= n; j++)                                    //What we basically do here is this:
            {                                               //We start from the top left corner. Then,
                if (i + j == n) //if element belongs to the main diagonal of the "sub-board"        //following the major "\" diagonal line of the main board,
                {//Observe: For all elements: i + j = n                         //we divide the board into "sub-boards" of size n.
                    sum += array[i][j]; //then add it to the general sum                //The desired sum is each time the sum of the sub-board's "/" major diagonal.
                }
            }
        }
        diagsum_l2r[n] = sum; //the sum of each diagonal goes to the array
    }

    for (n = 0; n <= 6; n++)  //same for the rest 7 diagonals
    {
        sum = 0;
        for (i = 7; i >= 0; i--)
        {
            for (j = 7; j >= 0; j--)
            {
                if (i + j == 14 - n)
                {
                    sum += array[i][j];
                }
            }
        }
        diagsum_l2r[14 - n] = sum;
    }

    for (n = 0; n <= 7; n++) //first 8 "/" diagonals (starting from the top left corner)
    {
        sum = 0;
        for (i = 7; i >= 7 - n; i--)
        {
            for (j = 0; j <= n; j++)
            {
                if (i - j == 7 - n)
                {
                    sum += array[i][j];
                }
            }
        }
        diagsum_r2l[n] = sum;
    }

    for (n = 0; n <= 6; n++) //last 7 "/" diagonals
    {
        sum = 0;
        for (i = 0; i <= 7; i++)
        {
            for (j = 7; j >= 0; j--)
            {
                if (j - i == 7 - n)
                {
                    sum += array[i][j];
                }
            }
        }
        diagsum_r2l[14 - n] = sum;
    }