C Program to Swap two Numbers

Given two numbers, write a C program to swap the given numbers.

Input : x = 10, y = 20;
Output : x = 20, y = 10

Input : x = 200, y = 100
Output : x = 100, y = 200

The idea is simple

1. Assign x to a temp variable : temp = x
2. Assign y to x : x = y
3. Assign temp to y : y = temp

Let us understand with an example.

x = 100, y = 200 After line 1: temp = x temp = 100 After line 2: x = y x = 200 After line 3 : y = temp y = 100

C

// C program to swap two variables
#include <stdio.h>

int main()
{
    int x, y;
    printf("Enter Value of x ");
    scanf("%d", &x);
    printf("\nEnter Value of y ");
    scanf("%d", &y);

    int temp = x;
    x = y;
    y = temp;

    printf("\nAfter Swapping: x = %d, y = %d", x, y);
    return 0;
}

Output:

Enter Value of x 12

Enter Value of y 14

After Swapping: x = 14, y = 12 

How to write a function to swap? Since we want the local variables of main to modified by swap function, we must them using pointers in C. 

C

// C program to swap two variables using a
// user defined swap()
#include <stdio.h>

// This function swaps values pointed by xp and yp
void swap(int *xp, int *yp)
{
    int temp = *xp;
    *xp = *yp;
    *yp = temp;
}

int main()
{
    int x, y;
    printf("Enter Value of x ");
    scanf("%d", &x);
    printf("\nEnter Value of y ");
    scanf("%d", &y);
    swap(&x, &y);
    printf("\nAfter Swapping: x = %d, y = %d", x, y);
    return 0;
}

Output:

Enter Value of x 12

Enter Value of y 14

After Swapping: x = 14, y = 12 

Auxiliary Space: O(1)

How to do in C++? In C++, we can use references also. 

CPP

// C++ program to swap two variables using a
// user defined swap()
#include <stdio.h>

// This function swaps values referred by
// x and y,
void swap(int &x, int &y)
{
    int temp = x;
    x = y;
    y = temp;
}

int main()
{
    int x, y;
    printf("Enter Value of x ");
    scanf("%d", &x);
    printf("\nEnter Value of y ");
    scanf("%d", &y);
    swap(x, y);
    printf("\nAfter Swapping: x = %d, y = %d", x, y);
    return 0;
}

Output:

Enter Value of x 12

Enter Value of y 14

After Swapping: x = 14, y = 12 

Is there a library function? We can use C++ library swap function also. 

CPP

// C++ program to swap two variables using a
// user defined swap()
#include <bits/stdc++.h>
using namespace std;

int main()
{
    int x, y;
    printf("Enter Value of x ");
    scanf("%d", &x);
    printf("\nEnter Value of y ");
    scanf("%d", &y);
    swap(x, y);
    printf("\nAfter Swapping: x = %d, y = %d", x, y);
    return 0;
}

Output:

Enter Value of x 12

Enter Value of y 14

After Swapping: x = 14, y = 12 

Swapping two numbers without using a temporary variable:

Approach: the simple idea behind this code is to use arithmetic operators. We will take the sum of the two numbers and store it in one number and store the difference of both the numbers in the other number. Finally we will store the difference of both the numbers in the first number. Since the values will be updated after the first two operations we will be able to swap the numbers. 

Algorithm:  

1) Take the input of the two numbers.                                                                                                                                               
2) Store the sum of both the numbers in the first number and store the difference of both the numbers in the second number. 
3) Finally store the difference of both the numbers in the first number and print both the numbers.

Code:

C

// C program to swap two numbers without using a third
// variable
#include <stdio.h>
int main()
{
    int a, b;
    printf("Enter the two numbers : \n");
    scanf("%d %d", &a, &b);
    printf("Before swapping A is : %d and B is %d \n", a,
           b);
    a = a + b;
    b = a - b;
    a = a - b;
    printf("After swapping A is : %d and B is : %d \n", a,
           b);
    return 0;
}

C++

// C++ program to swap two numbers without using a third
// variable
#include <iostream>
using namespace std;
int main()
{
    int a, b;
    cout << "Enter the two numbers : \n";
    cin >> a >> b;
    cout << "Before swapping A is : " << a
         << " and B is :  " << b << "\n";
    a = a + b;
    b = a - b;
    a = a - b;
    cout << "After swapping A is : " << a
         << " and B is :  " << b << "\n";
    return 0;
}

Output

Enter the two numbers : 
Before swapping A is : 0 and B is 32767 
After swapping A is : 32767 and B is : 0 

For all the above programs : 

Time Complexity: O(1)

Auxiliary Space: O(1), since no extra space has been taken.

How to swap without using a temporary variable?