Q.Write a Java program using a for loop to calculate and print the sum of the first N natural numbers, where N is entered by the user.

A running total is built up by adding each number as the loop counts from 1 to N. …

Complete program:

import java.util.Scanner;

public class SumN {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter N: ");
        int n = sc.nextInt();

        int sum = 0;                       // accumulator starts at 0
        for (int i = 1; i <= n; i++) {
            sum += i;                      // add each number to the total
        }
        System.out.println("Sum = " + sum);
    }
}

How it works. The variable sum is an accumulator, starting at 0. The for loop runs i from 1 up to n, and each pass adds the current i to sum. When the loop ends, sum holds 1 + 2 + ... + n.

Trace for N = 5:

The loop can be replaced by the direct formula sum = n * (n + 1) / 2;, which gives the same answer without iterating …

Not initialising sum to 0 (it must start empty). Using i < n instead of i <= n, which stops o …