Swapping two variable value without using third va

Question

One of the very tricky questions asked in an interview.

Swap the values of two variables like a=10 and b=15.

Generally to swap two variables values, we need 3rd variable like:

temp=a;
a=b;
b=temp;

Now the requirement is, swap values of two variables without using 3rd variable.

Answer 1

Here is one more solution but a single risk.

code:

#include <iostream>
#include <conio.h>
void main()
{

int a =10 , b =45;
*(&a+1 ) = a;
a =b;
b =*(&a +1);
}

any value at location a+1 will be overridden.

Answer 2

If you change a little the question to ask about 2 assembly registers instead of variables, you can use also the xchg operation as one option, and the stack operation as another one.

Answer 3

You may do....in easy way...within one line Logic

#include <stdio.h>

int main()
{
    int a, b;
    printf("Enter A :");
    scanf("%d",&a);
    printf("Enter B :");
    scanf("%d",&b);
    int a = 1,b = 2;
    a=a^b^(b=a);
    printf("\nValue of A=%d B=%d ",a,b);

    return 1;
}

or

#include <stdio.h>

int main()
{
    int a, b;
    printf("Enter A :");
    scanf("%d",&a);
    printf("Enter B :");
    scanf("%d",&b);
    int a = 1,b = 2;
    a=a+b-(b=a);
    printf("\nValue of A=%d B=%d ",a,b);

    return 1;
}

Answer 4

As already noted by manu, XOR algorithm is a popular one which works for all integer values (that includes pointers then, with some luck and casting). For the sake of completeness I would like to mention another less powerful algorithm with addition/subtraction:

A = A + B
B = A - B
A = A - B

Here you have to be careful of overflows/underflows, but otherwise it works just as fine. You might even try this on floats/doubles in the case XOR isn't allowed on those.

Answer 5

Swapping two numbers using third variable be like this,

int temp;
int a=10;
int b=20;
temp = a;
a = b;
b = temp;
printf ("Value of a", %a);
printf ("Value of b", %b);

Swapping two numbers without using third variable

int a = 10;
int b = 20;
a = a+b;
b = a-b;
a = a-b;
printf ("value of a=", %a);
printf ("value of b=", %b);

Answer 6

Of course, the C++ answer should be std::swap.

However, there is also no third variable in the following implementation of swap:

template <typename T>
void swap (T &a, T &b) {
    std::pair<T &, T &>(a, b) = std::make_pair(b, a);
}

Or, as a one-liner:

std::make_pair(std::ref(a), std::ref(b)) = std::make_pair(b, a);