C Program to Find Sum of N Natural Numbers

You can find the sum of n natural numbers in C by using a loop or formula; for example, sum = n * (n + 1) / 2; calculates the sum directly.

Examples

Input5

OutputSum of first 5 natural numbers is 15

Input10

OutputSum of first 10 natural numbers is 55

Input1

OutputSum of first 1 natural numbers is 1

How to Think About It

To find the sum of 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 sum quickly.

Algorithm

1

Get input value n from the user

2

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

3

Print the sum

Code

c

#include <stdio.h>

int main() {
    int n, sum;
    printf("Enter a positive integer: ");
    scanf("%d", &n);
    sum = n * (n + 1) / 2;
    printf("Sum of first %d natural numbers is %d\n", n, sum);
    return 0;
}

Output

Enter a positive integer: 5
Sum of first 5 natural numbers is 15

Dry Run

Let's trace the input 5 through the code

1

Input

User enters 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 entered by the user using scanf.

Step 2: Sum calculation

It uses the formula n * (n + 1) / 2 which adds all natural numbers from 1 to n efficiently.

Step 3: Output display

Finally, it prints the calculated sum using printf.

Alternative Approaches

Using a for loop

c

#include <stdio.h>

int main() {
    int n, sum = 0;
    printf("Enter a positive integer: ");
    scanf("%d", &n);
    for (int i = 1; i <= n; i++) {
        sum += i;
    }
    printf("Sum of first %d natural numbers is %d\n", n, sum);
    return 0;
}

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

Using recursion

c

#include <stdio.h>

int sumNatural(int n) {
    if (n == 1) return 1;
    return n + sumNatural(n - 1);
}

int main() {
    int n;
    printf("Enter a positive integer: ");
    scanf("%d", &n);
    printf("Sum of first %d natural numbers is %d\n", n, sumNatural(n));
    return 0;
}

This method uses recursion to add numbers but can cause stack overflow for very large n.

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

Time Complexity

Using the formula calculates the sum in constant time without loops.

Space Complexity

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

Which Approach is Fastest?

The formula method is fastest; loop and recursion take O(n) time and more memory.

Approach	Time	Space	Best For
Formula	O(1)	O(1)	Fastest and simplest
Loop	O(n)	O(1)	Easy to understand, good for beginners
Recursion	O(n)	O(n)	Demonstrates recursion but less efficient