C++ Program to Swap Two Numbers

In C++, you can swap two numbers using a temporary variable like this: int temp = a; a = b; b = temp; which exchanges the values of a and b.

Examples

Inputa = 5, b = 10

OutputAfter swap: a = 10, b = 5

Inputa = -3, b = 7

OutputAfter swap: a = 7, b = -3

Inputa = 0, b = 0

OutputAfter swap: 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. You store the first number in a temporary place, then replace the first number with the second, and finally put the stored number into the second variable.

Algorithm

1

Get the two numbers to swap.

2

Store the first number in a temporary variable.

3

Assign the second number to the first variable.

4

Assign the temporary variable's value to the second variable.

5

Print the swapped values.

Code

cpp

#include <iostream>
using namespace std;

int main() {
    int a = 5, b = 10;
    cout << "Before swap: a = " << a << ", b = " << b << endl;
    int temp = a;
    a = b;
    b = temp;
    cout << "After swap: a = " << a << ", b = " << b << endl;
    return 0;
}

Output

Before swap: a = 5, b = 10
After swap: a = 10, b = 5

Dry Run

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

1

Initial 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

Step	a	b	temp
Initial	5	10	-
Store a	5	10	5
Assign b to a	10	10	5
Assign temp to b	10	5	5

Why This Works

Step 1: Temporary storage

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

Step 2: Overwrite first variable

We assign the value of b to a, so now a has the second number.

Step 3: Restore second variable

We assign the stored value in temp to b, completing the swap.

Alternative Approaches

Using arithmetic operations

cpp

#include <iostream>
using namespace std;

int main() {
    int a = 5, b = 10;
    cout << "Before swap: a = " << a << ", b = " << b << endl;
    a = a + b;
    b = a - b;
    a = a - b;
    cout << "After swap: a = " << a << ", b = " << b << endl;
    return 0;
}

This method swaps without extra memory but can cause overflow if numbers are large.

Using std::swap function

cpp

#include <iostream>
#include <algorithm>
using namespace std;

int main() {
    int a = 5, b = 10;
    cout << "Before swap: a = " << a << ", b = " << b << endl;
    swap(a, b);
    cout << "After swap: a = " << a << ", b = " << b << endl;
    return 0;
}

This is the simplest and safest way using the standard library.

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

Time Complexity

Swapping two numbers takes constant time because it involves only a fixed number of operations.

Space Complexity

Only a small fixed amount of extra space is used for the temporary variable, so space complexity is constant.

Which Approach is Fastest?

All methods run in constant time; using std::swap is recommended for readability and safety.

Approach	Time	Space	Best For
Temporary variable	O(1)	O(1)	Simple and clear swaps
Arithmetic operations	O(1)	O(1)	No extra memory but risk of overflow
std::swap function	O(1)	O(1)	Clean, safe, and recommended