Write a C++ program that computes the sum of the first N natural numbers, where N is input by the user.

Input and Output Examples

• Input: 5
  Output: The sum of the first 5 natural numbers is 15.
• Input: 10
  Output: The sum of the first 10 natural numbers is 55.

Algorithm to calculate the sum of natural numbers:

1. Start the Program: Include necessary headers and declare the use of the standard namespace.
2. Prompt for Input: Ask the user to enter a positive integer N.
3. Read the Input: Store the input value in a variable.
4. Calculate the Sum: Use a for loop to start calculating the sum of the first N natural numbers from i = 1 to N.
5. Display the Output: Print the calculated sum.
6. End the Program: Finish execution cleanly.

Method 1: Using a Loop

This method uses a for loop to iterate from 1 to N, accumulating the sum of these numbers.

#include <iostream> 
using namespace std; 

int main() {
    int n, sum = 0; // Declare variables for the number of terms and the sum.

    // Take input va;ues
    cout << "Enter a positive integer: "; 
    cin >> n; // Read the integer value.

    // Calculate the sum using a for loop
    for (int i = 1; i <= n; ++i) {
        sum += i; // Add each number to sum.
    }

    cout << "The sum of the first " << n << " natural numbers is " << sum << "." << endl; // Display the sum.
    return 0; // End the program successfully.
}

Method 2: Using the Formula

For a more efficient calculation, use the mathematical formula for the sum of the first N natural numbers: (N×(N+1) ) / 2​.

#include <iostream>
using namespace std;

int main() {
    int n, sum; // Declare variables for the number of terms and the sum.
    cout << "Enter a positive integer: ";
    cin >> n;

    // Calculate the sum using the formula
    sum = n * (n + 1) / 2; // Apply the formula to calculate the sum.

    cout << "The sum of the first " << n << " natural numbers is " << sum << "." << endl;
    return 0;
}