C Program to Swap Two Numbers with Output and Explanation

You can swap two numbers in C by using a temporary variable like this: temp = a; a = b; b = temp; which exchanges the values of a and b.

Examples

Inputa = 5, b = 10

OutputAfter swapping: a = 10, b = 5

Inputa = -3, b = 7

OutputAfter swapping: a = 7, b = -3

Inputa = 0, b = 0

OutputAfter swapping: a = 0, b = 0

How to Think About It

To swap two numbers, think of holding one number temporarily so you don't lose it when you overwrite it. Use a temporary container to store one number, then replace it with the other, and finally put the stored number into the second variable.

Algorithm

1

Get input values for two numbers a and b

2

Store the value of a in a temporary variable temp

3

Assign the value of b to a

4

Assign the value of temp to b

5

Print the swapped values of a and b

Code

c

#include <stdio.h>

int main() {
    int a, b, temp;
    printf("Enter two numbers: ");
    scanf("%d %d", &a, &b);
    temp = a;
    a = b;
    b = temp;
    printf("After swapping: a = %d, b = %d\n", a, b);
    return 0;
}

Output

Enter two numbers: 5 10
After swapping: a = 10, b = 5

Dry Run

Let's trace swapping a=5 and b=10 through the code

1

Input values

a = 5, b = 10

2

Store a in temp

temp = 5

3

Assign b to a

a = 10

4

Assign temp to b

b = 5

5

Print swapped values

a = 10, b = 5

Variable	Value
a	5 -> 10
b	10 -> 5
temp	5

Why This Works

Step 1: Use a temporary variable

We use temp to hold the value of a so it is not lost when a is overwritten.

Step 2: Swap values

Assign b to a, then assign the stored temp value to b to complete the swap.

Step 3: Print result

Display the new values of a and b to confirm the swap.

Alternative Approaches

Swap without temporary variable using arithmetic

c

#include <stdio.h>

int main() {
    int a, b;
    printf("Enter two numbers: ");
    scanf("%d %d", &a, &b);
    a = a + b;
    b = a - b;
    a = a - b;
    printf("After swapping: a = %d, b = %d\n", a, b);
    return 0;
}

This method avoids extra memory but can cause overflow if numbers are large.

Swap using bitwise XOR operator

c

#include <stdio.h>

int main() {
    int a, b;
    printf("Enter two numbers: ");
    scanf("%d %d", &a, &b);
    a = a ^ b;
    b = a ^ b;
    a = a ^ b;
    printf("After swapping: a = %d, b = %d\n", a, b);
    return 0;
}

This method uses bitwise operations and no extra variable but is less readable.

Complexity: O(1) time, O(1) space

Time Complexity

Swapping two numbers uses a fixed number of operations, so it runs in constant time O(1).

Space Complexity

Only a few variables are used, so space complexity is constant O(1).

Which Approach is Fastest?

All methods run in O(1) time; using a temporary variable is simplest and safest, while arithmetic or XOR methods save memory but risk overflow or reduce readability.

Approach	Time	Space	Best For
Temporary variable	O(1)	O(1)	Clarity and safety
Arithmetic operations	O(1)	O(1)	Memory saving but risk overflow
Bitwise XOR	O(1)	O(1)	Memory saving, less readable