Swapping two variables(with and without using a third variable)

Swapping of variables is one of the most common type of operation in c++. It involves interchanging the values of two variables. It can be achieved by one of three ways.

1 - Using a third variable.

2 - Without using a third variable

1- Swapping using a third variable : In this method a third variable is used to hold the value of one of the variables.Suppose we want to swap the values of two integers, say x and y. To do that , we would first declare a third variable, say temp. Next, assign the value of x to temp so we have to copies of x , one in x and another in temp. 

Then, assign the value of y to x. This will overwrite the value of x with y.

Now, assign the value of x to y, using temp, which contains the original value of x, and there we have it. Swap successful !!!! The following program shows the above procedure :

// Program to swap two numbers
#include <iostream>

int main()
{
    int x,y,temp;
    std::cout<<" Enter the value of x : ";
    std::cin>>x;
    std::cout<<"\n Enter the value of y : ";
    std::cin>>y;
    std::cin.ignore();                  // to clear the buffer
    std::cout<<"\n Value of x and y before swapping : "<<x<<" "<<y;
    // swaps the two variables x and y using the third variable temp
    temp = x;           
    x = y;
    y = temp;
    std::cout<<"\n Value of x and y after swapping : "<<x<<" "<<y;
    std::cin.get();      // to prevent the console from closing immediately
    return 0;
} // end of main 

Swapping using a third variable

 2 - Swapping using WITHOUT using a third variable : This is a very simple concept to understand. First add both the variables, then subtract two times. For e.g. , let us swap two variables x and y each having values 10 and 5 respectively. 

Then, according to swap the values , firstly

x = x + y; // adding x and y and assigning the result to x ie x = 10 + 5 i.e x = 15;

y = x - y; // subtracting  x and y and assigning it to y i.e y = 10 - 5 which is 5, which is also the original value of x, hence y now has the original value of x.

finally x = x - y; // i.e 15 - 5 = 10, hence x = 10, which is the original value of y.

Who la, we have swapped the two value.

NOTE : This method is only useful when x > y and will not work the other way round.

A program is given below to demonstrate the above method :
 // Program to swap two numbers(without using a third variable)
#include <iostream>

int main()
{
    int x,y;
    std::cout<<" Enter the value of x : ";
    std::cin>>x;
    std::cout<<"\n Enter the value of y : ";
    std::cin>>y;
    std::cin.ignore(); // to clear the buffer
    std::cout<<"\n Value of x and y before swapping : "<<x<<" "<<y;
    x = x + y;        
    y = x - y;
    x = x - y;
    std::cout<<"\n Value of x and y after swapping : "<<x<<" "<<y;
    std::cin.get();      // to prevent the console from closing immediately
    return 0;
} // end of main

Swapping without using a third variable