C Program to Calculate Sum of N Natural Numbers

C Programming Examples

C Number Programs

This tutorial will guide you on how to write a C program to compute the sum of the first N natural numbers. It's a common problem for beginners that helps you understand basic loops and mathematical operations.



What are Natural Numbers?

Natural numbers are positive integers beginning with 1 (e.g., 1, 2, 3, and so on). There are two methods to calculate the sum of the first N natural numbers:

  1. Using a loop: This method involves iterating from 1 to N and adding each number to a running total.
  2. Using a mathematical formula: This method uses a direct formula for a faster calculation.

Calculate Sum of N Natural Numbers (Using a Loop)

Here's a simple C program that uses a for loop to calculate the sum of N natural numbers. The program prompts the user to enter a value for N, and then it iterates from 1 to N, adding each number to a running sum.

Example:

#include <stdio.h>

int main() {
    int n, sum = 0;

    // Ask the user for a positive integer input and validate it
    do {
        printf("Please enter a positive integer: ");
        scanf("%d", &n);

        if (n <= 0) {
            printf("Invalid input. The number must be a positive integer.\n");
        }
    } while (n <= 0);

    // Iterate from 1 to n, adding each number to the sum
    for (int i = 1; i <= n; i++) {
        sum += i;  // Add the current value of i to sum
    }

    // Print the final result
    printf("The sum of the first %d natural numbers is: %d\n", n, sum);

    return 0;
}

Output:

Please enter a positive integer: -3
Invalid input. The number must be a positive integer.
Please enter a positive integer: 5
The sum of the first 5 natural numbers is: 15

Explanation:

  • User Input: The program asks the user for a number, which is stored in the variable n.
  • Input Validation: The program ensures that the user inputs a positive integer. If the input is invalid (like a negative number or zero), it prompts the user again.
  • Looping Through Numbers: The for loop runs from 1 to n. In each iteration, the current number i is added to sum.
  • Print the Sum: Once the loop is complete, the program prints the final value of sum.

Calculate Sum of N Natural Numbers (Using Formula)

Instead of using a loop, you can calculate the sum in one step using a direct formula. This method is faster, especially for larger numbers. The formula for calculating the sum of the first N natural numbers is:

Formula:

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

Example:

#include <stdio.h>

int main() {
    int n, sum;

    // Ask the user for a positive integer input and validate it
    do {
        printf("Please enter a positive integer: ");
        scanf("%d", &n);

        if (n <= 0) {
            printf("Invalid input. The number must be a positive integer.\n");
        }
    } while (n <= 0);

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

    // Output the result
    printf("The sum of the first %d natural numbers is: %d\n", n, sum);

    return 0;
}

Output:

Please enter a positive integer: 0
Invalid input. The number must be a positive integer.
Please enter a positive integer: 10
The sum of the first 10 natural numbers is: 55

Explanation:

  • User Input: The user enters a number n.
  • Input Validation: The program ensures the input is a valid positive integer before performing the calculation.
  • Formula Calculation: The sum is calculated directly using the formula Sum = (N * (N + 1)) / 2.
  • Print the Sum: The result is printed immediately.

Conclusion

This tutorial demonstrated two efficient methods for calculating the sum of the first N natural numbers in C. By understanding both the loop and formula approaches, you can select the best method based on your specific needs and the size of the input.



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