Floyd Triangle Example in Java

Introduction

Floyd Triangle is a classic Java console program that demonstrates the concept with complete source code and sample output. Pattern programs print shapes with nested loops — common in exams and interviews.

This tutorial walks through the program line by line, explains how the logic works, and highlights best practices you can apply in your own code.

Definition

Floyd’s triangle is a right-angled triangular array of natural numbers, used in computer science education. It is named after Robert Floyd. It is defined by filling the rows of the triangle with consecutive numbers, starting with a 1 in the top left corner

Formula

The nth row sums to n(n^2 + 1)/2

Floyd Triangle Example Program

import java.util.Scanner;
 
class FloydTriangle{
	public static void main(String args[]){
		int count, num = 1;
		Scanner in = new Scanner(System.in);
		System.out.print("Enter the number of rows:");
		count = in.nextInt();
		for (int i = 1 ; i <=count ; i++ ){
			for (int j = 1 ; j <= i ; j++ ){
				System.out.print(num+" ");
				num++;
			}
        System.out.println();
		}
	}
}

Sample Output

Enter the number of rows:7
1
2 3
4 5 6
7 8 9 10
11 12 13 14 15
16 17 18 19 20 21
22 23 24 25 26 27 28

When to use

Use pattern logic when practicing nested loops or preparing for coding tests that ask for triangle or pyramid output.

How it works

1. Execution begins in the main method — the JVM calls this method when you run the class.

2. import java.util.Scanner; imports a class used later in the program.

3. int count, num = 1; updates a variable used in the calculation or output.

4. A Scanner reads typed input from the keyboard (System.in).

5. A println / print call writes text to the console — part of the sample output below.

6. count = in.nextInt(); updates a variable used in the calculation or output.

7. for (int i = 1 ; i <=count ; i++ ){ updates a variable used in the calculation or output.

8. A println / print call writes text to the console — part of the sample output below.

Best Practices

• Use meaningful variable and class names that describe their purpose.
• Compile and run the program locally — modify values to see how output changes.
• Read compiler errors carefully; they usually point to the exact line to fix.

Common Mistakes

• Copying code without understanding each line — practice by changing one statement at a time.
• Mismatching the public class name and the .java filename.
• Forgetting semicolons at the end of statements.