// IN:  x and y are NUMDIG arrays containing Big Integers
// OUT:  sum is a NUMDIG2 array containing the arithmetic sum
const int NUMDIG = 20, NUMDIG2 = 21;
void SumBigInt (int x[ ], int y[ ], int sum[ ])
{	int n, carry=0;
	for (n=NUMDIG-1; n>=0; n--)
	{	sum[n+1] = x[n] + y[n] + carry;
		if (sum[n+1] >= 10)
		{	sum[n+1] -= 10;			// perform long addition
			carry = 1;				// digit-by-digit in the arrays
		}
		else carry = 0;
	}
	sum[0] = carry;
}