C Program to Swap Two Numbers using Pointers

In this article, we will write a C program to swap two numbers using pointers.

The program takes two numbers from the user as input, stores these numbers in variables a and b, and swaps their values after executing.

Sample Input:

Enter the value of a and b: 10 20

Output:

Before swapping a = 10 and b = 20
After swapping a = 20 and b = 10

Swapping is the process of interchanging the values of two variables. In other words, swapping the values of two variables means that the value of the first variable is assigned to the second variable and the value of the second variable is assigned to the first variable.

In C programming, there are different ways to swap two numbers. We will use C pointers to swap two numbers.

In C language, pointers are used to store the memory address of other variables. They are declared by putting an asterisk(*) symbol before the variable name, for eg. *myPtr.

When we assign a variable to a pointer, the pointer holds the memory address of that variable. So, if we make any changes to the variable, these changes directly reflect on the pointer or vice versa.

The following C program shows how we can swap two numbers using pointers:

// C program to swap two numbers using pointers
#include <stdio.h>

int main() {

    int a, b, temp;
    int *ptr1, *ptr2;

    printf("Enter the value of a and b: ");
    scanf("%d %d", &a, &b);

    printf("\nBefore swapping a = %d and b = %d", a, b);

    // Assign the memory address of a and b to *ptr1 and *ptr2
    ptr1 = &a;
    ptr2 = &b;

    // Swap the values a and b
    temp = *ptr1;
    *ptr1 = *ptr2;
    *ptr2 = temp;

    printf("\nAfter swapping a = %d and b = %d", a, b);

    return 0;

}

Output:

Enter the value of a and b: 10 20

Before swapping a = 10 and b = 20
After swapping a = 20 and b = 10

Code Explanation:

• The program takes two numbers from the user and stores them in num1 and num2 variables respectively.
• We then store the memory address of num1 in *ptr1 and the memory address of num2 in *ptr2 using the statements ptr1 = &a; and ptr2 = &b; respectively.
• The pointers *ptr1 and *ptr2 are now directly pointing to variables a and b respectively, therefore, if we make any changes to these pointers, the changes will also reflect on variables a and b.
• To swap the values of a and b, we have created a temporary variable temp. The variable temp is used to hold the value of *ptr1 i.e. value of variable a.
• The statement *ptr1 = *ptr2; means that the value of *ptr2 is assigned to *ptr1.
• Finally, we copied the old value of *ptr1 to *ptr2 using the temp variable i.e. *ptr2 = temp;.

Example 2: Swap Two Numbers by Passing Pointers to Functions

We can also create a user-defined function which will take the references of the two variables as its arguments and swap their values with the help of pointers.

In the following C program, we have created a function swap(). The swap() function takes two arguments which are actually the memory addresses of the first and second variables and performs swapping on calling.

// C program to swap two numbers using pointers
#include <stdio.h>

// Function declaration
void swap(int * , int * );

int main() {

  int a, b, temp;
  int *ptr1, *ptr2;

  printf("Enter the value of a and b: ");
  scanf("%d %d", &a, &b);

  printf("\nBefore swapping a = %d and b = %d", a, b);

  // Call the swap() function to swap a and b
  swap( &a, &b);

  printf("\nAfter swapping a = %d and b = %d", a, b);

  return 0;

}

// User-defined function to swap two numbers
void swap(int * ptr1, int * ptr2) {
  int temp;

  temp = * ptr1;
  * ptr1 = * ptr2;
  * ptr2 = temp;

}

Output:

Enter the value of a and b: 10 20

Before swapping a = 10 and b = 20
After swapping a = 20 and b = 10

Thanks for reading.