Swapping 2 variables without 3rd variable

Method1 (The XOR trick)

a ^= b ^= a ^= b;


Although the code above works fine for most of the cases, it tries to modify variable 'a' two times between sequence points, so the behavior is undefined. What this means is it wont work in all the cases. This will also not work for floating-point values. Also, think of a scenario where you have written your code like this



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


Now, if suppose, by mistake, your code passes the pointer to the same variable to this function. Guess what happens? Since Xor'ing an element with itself sets the variable to zero, this routine will end up setting the variable to zero (ideally it should have swapped the variable with itself). This scenario is quite possible in sorting algorithms which sometimes try to swap a variable with itself (maybe due to some small, but not so fatal coding error). One solution to this problem is to check if the numbers to be swapped are already equal to each other.


swap(int *a, int *b)
{
  if(*a!=*b)
  {
    *a ^= *b ^= *a ^= *b;
  }
}




Method2

This method is also quite popular


 a=a+b;
 b=a-b;
 a=a-b;


But, note that here also, if a and b are big and their addition is bigger than the size of an int, even this might end up giving you wrong results.