Swapping two variable value without using third variable in C/C++

In C programming, swapping two variables without using a third variable can be accomplished using arithmetic operations. This technique saves memory and demonstrates clever use of mathematical operations.

Syntax

a = a + b;
b = a - b;
a = a - b;

Method 1: Using Addition and Subtraction

This approach uses arithmetic operations to swap values without requiring additional memory −

#include <stdio.h>

int main() {
    int a = 10, b = 20;
    
    printf("Before swapping: a = %d, b = %d\n", a, b);
    
    a = a + b;  // a becomes 30 (10 + 20)
    b = a - b;  // b becomes 10 (30 - 20)
    a = a - b;  // a becomes 20 (30 - 10)
    
    printf("After swapping: a = %d, b = %d\n", a, b);
    return 0;
}

Before swapping: a = 10, b = 20
After swapping: a = 20, b = 10

Method 2: Using XOR Operation

The XOR bitwise operation provides another efficient way to swap variables −

#include <stdio.h>

int main() {
    int a = 15, b = 25;
    
    printf("Before swapping: a = %d, b = %d\n", a, b);
    
    a = a ^ b;  // XOR of a and b
    b = a ^ b;  // XOR again to get original a
    a = a ^ b;  // XOR to get original b
    
    printf("After swapping: a = %d, b = %d\n", a, b);
    return 0;
}

Before swapping: a = 15, b = 25
After swapping: a = 25, b = 15

Method 3: Using Single Line Expression

All operations can be combined into a single line using the comma operator −

#include <stdio.h>

int main() {
    int a = 50, b = 75;
    
    printf("Before swapping: a = %d, b = %d\n", a, b);
    
    a = a + b - (b = a);  // Single line swap
    
    printf("After swapping: a = %d, b = %d\n", a, b);
    return 0;
}

Before swapping: a = 50, b = 75
After swapping: a = 75, b = 50

Comparison

Key Points

• These methods work only with numeric data types.
• XOR method is safest as it avoids overflow issues.
• For production code, using a temporary variable is often clearer and safer.

Conclusion

Swapping without a third variable demonstrates efficient memory usage in C. While these techniques are clever, they should be used carefully considering readability and potential overflow issues.