C Program to Swap two Numbers

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

Example:

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

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

Using Temporary Variable

Below is the C program to swap two numbers using a temporary variable:

C

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

// Driver code
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 

Time Complexity: O(1)
Auxiliary Space: O(1)

Using Function

Since we want the local variables of the main to be modified by the swap function, we must them using pointers in C. Below is the C program to swap two numbers using pointers:

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;
}

// Driver code
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 

Time Complexity: O(1)
Auxiliary Space: O(1)

Using References

Below is the C++ program to swap two numbers using references:

C++

// 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;
}

// Driver code
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 

Time Complexity: O(1)
Auxiliary Space: O(1)

Using Library swap() Function

Below is the C++ program to swap two numbers using the C++ library swap function:

C++

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

// Driver code
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 

Time Complexity: O(1)
Auxiliary Space: O(1)

Without Using a Temporary Variable

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 numbers in the other number. Finally, we will store the difference of both 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.

Below is the C/C++ program to swap two numbers without using a temporary variable:

C

// C program to swap two numbers
// without using a third variable
#include <stdio.h>

// Driver code
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;

// Driver code
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 

Time Complexity: O(1)

Auxiliary Space: O(1)

Using XOR

The simple idea behind this code is the fact that XOR-ing a value with itself results in 0.

Algorithm:  

1. Take two variables a and b.
2. XOR a with b, and store the result in a.
3. XOR b with the new value of a, and store the result in b.
4. XOR a with the new value of b, and store the result in a.
5. The values of a and b are now swapped.

Below is the C++ program to swap two numbers using XOR:

C++

// C++ program to swap two
// numbers using XOR variable
#include <iostream>
using namespace std;

// Driver code
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

How to swap without using a temporary variable?