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

Outputa = 10, b = 5

Inputa = -3, b = 7

Outputa = 7, b = -3

Inputa = 0, b = 0

Outputa = 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 its place. 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 the two numbers to swap, call them 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 stored in temp to b.

5

Now a and b have swapped values.

Code

csharp

using System;
class Program {
    static void Main() {
        int a = 5, b = 10;
        Console.WriteLine($"Before swap: a = {a}, b = {b}");
        int temp = a;
        a = b;
        b = temp;
        Console.WriteLine($"After swap: a = {a}, b = {b}");
    }
}

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

Variable	Value
a	5
b	10
temp	5
a	10
b	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 we overwrite a.

Step 2: Assign b to a

We replace a with b's value, so now a has the second number.

Step 3: Assign temp to b

Finally, we put the original a value stored in temp into b, completing the swap.

Alternative Approaches

Using arithmetic operations

csharp

using System;
class Program {
    static void Main() {
        int a = 5, b = 10;
        Console.WriteLine($"Before swap: a = {a}, b = {b}");
        a = a + b;
        b = a - b;
        a = a - b;
        Console.WriteLine($"After swap: a = {a}, b = {b}");
    }
}

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

Using tuple deconstruction (C# 7.0+)

csharp

using System;
class Program {
    static void Main() {
        int a = 5, b = 10;
        Console.WriteLine($"Before swap: a = {a}, b = {b}");
        (a, b) = (b, a);
        Console.WriteLine($"After swap: a = {a}, b = {b}");
    }
}

This is the cleanest and most modern way but requires C# 7.0 or later.

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; tuple deconstruction is cleanest, arithmetic uses no extra variable but risks overflow.

Approach	Time	Space	Best For
Temporary variable	O(1)	O(1)	Simple and safe for all numbers
Arithmetic operations	O(1)	O(1)	No extra variable but risky with large numbers
Tuple deconstruction	O(1)	O(1)	Cleanest syntax, requires C# 7.0+