C Program – Calculate Sum of Natural Numbers - Loop & Formula

C Basics

Sum of Natural Numbers Using Loop

#include <stdio.h>
int main() {
    int N, i;
    int sum = 0;
    printf("Enter a positive integer: ");
    scanf("%d", &N);
    for (i = 1; i <= N; i++) {
        sum += i;
    }
    printf("Sum = %d\n", sum);
    return 0;
}
Enter a positive integer: 5
Sum = 15

This method iterates from 1 up to the user-specified limit and adds each number to an accumulator variable. The result is printed after the loop completes.

Sum of Natural Numbers Using Formula

#include <stdio.h>
int main() {
    int N;
    int sum;
    printf("Enter a positive integer: ");
    scanf("%d", &N);
    sum = N * (N + 1) / 2;
    printf("Sum = %d\n", sum);
    return 0;
}
Enter a positive integer: 10
Sum = 55

The formula N × (N + 1) / 2 provides a constant-time way to compute the sum of the first N natural numbers without iteration. This is especially useful for large values of N.


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