Question

I hope someone is able to help me.

I have the following algorithm that works both if I have two positive numbers or a positive and a negative number. It doesn’t works, however, if both numbers are negative.

Can someone explain me how is it possible?

void sum (int p, int q) {

int sum, carry;

carry = 1;

while (carry > 0) {
    sum = p ^ q;
    carry = p & q;
    carry = carry << 1;
    p = sum;
    q = carry;
}

p = p << 1;
p = p >> 1;

printf("The result equals to %d", p);

}

Thanks in advance to everyone 🙂

Answer 1

The sum of two negative numbers is negative (unless an overflow occurs, as pointed out in a comment). But this code:

p = p << 1;
p = p >> 1;

is essentially clearing the topmost bit, i.e. the sign bit, so the result of this will never be negative.