How to swapping two variables with a pointer

Question

I have a problem, I can't create a simple program for swapping two variables with a pointer. I tried in this way:

int main() {

int x=5, y=10;
int *p=nullptr;

p=&x;
y=*p;
x=y;

cout<<x<<" "<<y;
}

How can I do?

Answer 1

void swap(int *x, int *y)
{
    int tmp;

    tmp = *x;
    *x = *y;
    *y = tmp;
}
int main()
{
    int a = 7, b = 3;

    std::cout << a << ' ' << b << '\n';
    swap(&a, &b);
    std::cout << a << ' ' << b << '\n';
}

Also consider std::swap

Answer 2

The problem is that you are overwriting the original value of y with the value of x , then copying that value back to x .

What you want to do is to stash the value of x , copy the value of y to it, then move the stashed value to y .

Since you are trying to do this with pointers, you should probably change the operation x=y to use pointers as well. Here is an example:

int main(int argc, char *argv[])
{
    int x=5, y=10, stash;
    int *px, *py;

    cout << "Before: x=" << x << " y=" << y;

    px = &x;
    py = &y;

    stash = *px;
    *px = *py;
    *py = stash;

    cout << "After: x=" << x << " y=" << y;
}

Answer 3

Here's another good old way of swapping values of two variables without using a third variable:

x = x + y;  // x now becomes 15
y = x - y;  // y becomes 10
x = x - y;  // x becomes 5

Yet another way of doing it, as already pointed out by someone is using xor .

x = x ^ y;  // x now becomes 15 (1111)
y = x ^ y;  // y becomes 10 (1010)
x = x ^ y;  // x becomes 5 (0101)

There is also a one liner way of doing it:

x = x + y - (y = x);

You don't necessarily need pointers.

Limitation - As all the commentators have indicated, there is a limitation with all these approaches. These approaches may give you incorrect results if the sum of x and y exceeds INT_MAX (which may be equal to 2,147,483,647 on positive side and -2,147,483,648 on a negative side on a 32 bit system) AND your C implementation doesn't wrap around signed integers. However for your program, if you are working with relatively smaller numbers or your C implementation wraps around signed integers too, you should not have any issues using any of these 3 methods. But if you think that the sum might exceed INT_MAX , you can use the old fashioned temp variable approach to swap the values or use the standard swap function provided in the libraries.