Definition of Triangular Number

Triangular number of a natural number n is the sum of all natural numbers from 1 to n. For example, Triangular number of 5 = 1 + 2 + 3 + 4 + 5 = 15.  Mathematically,

It is known as triangular numbers since it is the total number of dots in a triangle with n dots on a side. For example, Triangular number of 5 can be represented in the following triangle,

Finding Triangular Number of a Number in Java

The following Java program finds the triangular number of a given number. It uses a simple loop to find the sum,

/* Printing Triangular number for a number in Java */
public class TriangularNumber {
    
    public static void main(String[] args) {
       int number = 6;
       int triangular = 0;
       for(int i = 1;i<=6;i++) {
           triangular = triangular + i;
       }
       System.out.println("Triangular Number for "+number+" is "+triangular);
    }
    
}

Finding Triangular Numbers of a Range of Numbers

The following Java program finds triangular numbers for a range of natural numbers,

/* Printing Triangular number for a range of numbers in Java */

public class TriangularNumbers {

    public static void main(String[] args) {
        int starting_number = 1;
        int ending_number = 10;

        System.out.println("List of Triangular Numbers ");
        for (int i = starting_number; i <= ending_number; i++) {
            int triangular = 0;


            for (int j = 1; j <= i; j++) {
                triangular = triangular + j;
            }
            System.out.println(i + " = " + triangular);
        }
    }
}

Another way of calculating Triangular number is to use the following formula,

Triangular number of n = (n*(n+1))/2.

/* Printing Triangular number for a number in Java using n*(n+1)/2 formula */
public class AlternateTriangularNumber {
    
    public static void main(String[] args) {
       int number = 9;
       int triangular = number * (number+1)/2;
       System.out.println("Triangular Number for "+number+" is "+triangular);
    }
    
}