Java Program to Display Numbers and Sum of First N Natural Numbers

Print first N natural numbers using an iterative approach i.e. using for loop. For loop has three parameters initialization, testing condition, and increment/decrement.

Input:     N = 10
Output:    First 10 Numbers = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
           Sum of first 10 Natural Number = 55

Input:     N = 5
Output:    First 5 Numbers = 1, 2, 3, 4, 5
           Sum of first 5 Natural Number = 15

Approach

1. Start for loop initialization with i = 1.
2. Write testing condition as i <= N.
3. Add increment statement as i++ or i+=1.
4. Initialize a variable sum with 0.
5. Start adding i with the sum at each iteration of for loop and print i.
6. Print sum at the end for loop. 
   

Below is the implementation of above approach

Java

// Java Program to Display Numbers
// from 1 to N Using For Loop and
// sum of First N Natural Number
import java.io.*;
class GFG {
    public static void main(String[] args)
    {
        int N = 10;
        int sum = 0;
        System.out.print("First " + N + " Numbers = ");

        // we initialize the value of the variable i
        // with 1 and increment each time with 1
        for (int i = 1; i <= N; i++) {

            // print the value of the variable as
            // long as the code executes
            System.out.print(i + " ");
            sum += i;
        }
        System.out.println();
        System.out.println("Sum of first " + N
                           + " Natural Number = " + sum);
    }
}

Output

First 10 Numbers = 1 2 3 4 5 6 7 8 9 10 
Sum of first 10 Natural Number = 55

Time Complexity: O(n)
Auxiliary Space: O(1) because constant space for variables is being used

Alternate Approach

1. Start for loop initialization with i = 1.
2. Write testing condition as i <= N.
3. Add increment statement as i++ or i+=1.
4. Start Printing i for each iteration.
5. Print sum using first N natural number formula at the end of for loop.

Below is the implementation of the above approach

Java

// Java Program to Display Numbers
// from 1 to N Using For Loop and
// sum of First N Natural Number
import java.io.*;

class GFG {

    public static void main(String[] args)
    {
        int N = 5;
        System.out.print("First " + N + " Numbers = ");

        // we initialize the value of the variable i
        // with 1 and increment each time with 1
        for (int i = 1; i <= N; i++) {

            // print the value of the variable as
            // long as the code executes
            System.out.print(i + " ");
        }
        System.out.println();
        System.out.println("Sum of first " + N
                           + " Natural Number = " + (N*(N+1))/2);
    }
}

Output

First 5 Numbers = 1 2 3 4 5 
Sum of first 5 Natural Number = 15

Time Complexity: O(n)
Auxiliary Space: O(1) as it is using constant space for variables