C# Program to Find Sum of N Natural Numbers

You can find the sum of n natural numbers in C# using int sum = n * (n + 1) / 2; or by looping from 1 to n and adding each number.

Examples

Input1

OutputSum of first 1 natural numbers is 1

Input5

OutputSum of first 5 natural numbers is 15

Input10

OutputSum of first 10 natural numbers is 55

How to Think About It

To find the sum of the first n natural numbers, think of adding all numbers from 1 up to n. You can do this by adding each number one by one or use the formula n * (n + 1) / 2 which gives the total directly.

Algorithm

1

Get the input number n from the user

2

Calculate the sum using the formula n * (n + 1) / 2

3

Print the result

Code

csharp

using System;
class Program {
    static void Main() {
        Console.Write("Enter a number: ");
        int n = int.Parse(Console.ReadLine() ?? "0");
        int sum = n * (n + 1) / 2;
        Console.WriteLine($"Sum of first {n} natural numbers is {sum}");
    }
}

Output

Enter a number: 5
Sum of first 5 natural numbers is 15

Dry Run

Let's trace the input 5 through the code

1

Input

User enters 5, so n = 5

2

Calculate sum

sum = 5 * (5 + 1) / 2 = 5 * 6 / 2 = 30 / 2 = 15

3

Output

Prints 'Sum of first 5 natural numbers is 15'

n	sum
5	15

Why This Works

Step 1: Input reading

The program reads the number n from the user using Console.ReadLine() and converts it to an integer.

Step 2: Sum calculation

It uses the formula n * (n + 1) / 2 which adds all natural numbers from 1 to n efficiently without looping.

Step 3: Output display

The program prints the sum with a clear message using string interpolation $"...".

Alternative Approaches

Using a for loop

csharp

using System;
class Program {
    static void Main() {
        Console.Write("Enter a number: ");
        int n = int.Parse(Console.ReadLine() ?? "0");
        int sum = 0;
        for (int i = 1; i <= n; i++) {
            sum += i;
        }
        Console.WriteLine($"Sum of first {n} natural numbers is {sum}");
    }
}

This method uses a loop to add each number one by one, which is easy to understand but slower for very large n.

Using recursion

csharp

using System;
class Program {
    static int SumNatural(int n) {
        if (n == 1) return 1;
        return n + SumNatural(n - 1);
    }
    static void Main() {
        Console.Write("Enter a number: ");
        int n = int.Parse(Console.ReadLine() ?? "0");
        int sum = SumNatural(n);
        Console.WriteLine($"Sum of first {n} natural numbers is {sum}");
    }
}

This method uses recursion to add numbers, which is elegant but can cause stack overflow for large n.

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

Time Complexity

Using the formula, the calculation is done in constant time without loops.

Space Complexity

Only a few variables are used, so space is constant.

Which Approach is Fastest?

The formula approach is fastest; looping or recursion take O(n) time and more memory.

Approach	Time	Space	Best For
Formula	O(1)	O(1)	Fastest calculation for any n
For loop	O(n)	O(1)	Simple logic, easy to understand
Recursion	O(n)	O(n)	Elegant but risky for large n