Why am I getting zero in the output?

So I'm trying to swap two numbers without using a third variable. I pass the two numbers I'm trying to swap by reference. When the same number is to be swapped (like 1 is to be swapped with 1), I get an output of zero. I know how to fix this, but I'm unable to understand as to why I keep getting 0 when a and b are the same.

Can anyone explain why I'm getting zero instead of the swapped numbers?

void swap(int *a,int *b)
{
        //if(a==b)
        //      return;
        *a=*a+*b;
        *b=*a-*b;
        *a=*a-*b;
}
int main()
{
    int n=3,x=0,y=0;
    int a[n][n];
    for(int i=0;i<n;i++)
                swap(&a[x][i],&a[i][y]);
    return 0;
}

Solution

It seems you are trying to swap elements of the first row with elements of the first column.

For starters variable length arrays is not a standard C++ feature. Though some compilers have their own language extensions you should avoid their using.

Also the swap function can have undefined behavior because when there is an overflow of a signed integer then the result is undefined. It is better to use the standard C++ function std::swap. And in your program you are using an uninitialized array.

Here is a demonstrative program that shows how you could write the code

#include <iostream>
#include <utility>

int main() 
{
    const size_t N = 3;
    int a[N][N];

    for ( size_t i = 0; i < N; i++ )
    {
        for ( size_t j = 0; j < N; j++ )
        {
            a[i][j] = i * N + j;
        }
    }

    for ( const auto &row : a )
    {
        for ( const auto &item : row )
        {
            std::cout << item << ' ';
        }
        std::cout << '\n';
    }

    std::cout << '\n';

    for ( size_t i = 0; i < N; i++ )
    {
        std::swap( a[0][i], a[i][0] );
    }

    for ( const auto &row : a )
    {
        for ( const auto &item : row )
        {
            std::cout << item << ' ';
        }
        std::cout << '\n';
    }

    std::cout << '\n';

    return 0;
}

Its output is

If you want to write your own swap function then write it like

void swap( int *a, int *b )
{
    int tmp = *a;
    *a = *b;
    *b = tmp;
}

and use it in the program like

::swap( &a[0][i], &a[i][0] );