Sum of first N natural numbers in C

You can calculate the sum of the first N natural numbers (positive integers) in C using a simple loop. The sum of the first N natural numbers is given by the formula:

Sum = 1 + 2 + 3 + ... + N = N * (N + 1) / 2

Here’s a C program to calculate the sum of the first N natural numbers:

#include <stdio.h>

int main() {
    int N;
    long long sum = 0; // Use a long long to handle larger values

    printf("Enter a positive integer N: ");
    scanf("%d", &N);

    if (N < 1) {
        printf("N must be a positive integer.\n");
    } else {
        // Calculate the sum using the formula
        sum = (long long)N * (N + 1) / 2;

        printf("Sum of the first %d natural numbers is %lld\n", N, sum);
    }

    return 0;
}

In this program:

• We prompt the user to enter a positive integer N.
• We check if N is less than 1 (i.e., not a positive integer) and provide an error message if it’s not.
• If N is a positive integer, we calculate the sum using the formula sum = (long long)N * (N + 1) / 2 and then display the result.

Note that we use a long long data type for the sum variable to handle larger values of N without overflowing.

Compile and run the program, and it will calculate and display the sum of the first N natural numbers based on the user’s input.