Java Program to Print Floyd’s Triangle with Example

Floyd’s triangle is a right-angled triangle of natural numbers, which 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, there are n integers in the nth row and a sum of (n(n+1))/2 integers in n rows. Write a java program to print Floyd’s triangle is one of the popular java interview questions. 

Floyd’s Triangle in java

package com.javainterviewpoint;

import java.util.Scanner;

/**
 * @author JavaInterviewPoint
 *
 */
public class FloydTriangle
{
    
    /**
     * @param args
     */
    public static void main(String[] args)
    {
        //Create a new Scanner object
        Scanner scanner = new Scanner(System.in);
        
        //Get the number of rows from the user
        System.out.println("Enter the number of row to print");
        int rows = scanner.nextInt();
        
        int floydNumber=1;
        System.out.println("** Printing **");
        for(int i=1;i<=rows;i++)
        {
            for(int j=1;j<=i;j++)
            {
                System.out.print(floydNumber+" ");
                floydNumber = floydNumber + 1;
            }
            System.out.println();
        }
    }
}

• Get the number of rows which needs to be printed from the user
• We have two loops, the first loop is used for the number of the rows and second loop is for printing the floydNumber and incrementing it by 1

Output

Enter the number of row to print
9
** Printing **
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 
29 30 31 32 33 34 35 36 
37 38 39 40 41 42 43 44 45

Floyd Triangle with stars in Java

package com.javainterviewpoint;

import java.util.Scanner;

/**
 * @author JavaInterviewPoint
 *
 */
public class FloydTriangle
{
    
    /**
     * @param args
     */
    public static void main(String[] args)
    {
        //Create a new Scanner object
        Scanner scanner = new Scanner(System.in);
        
        //Get the number of rows from the user
        System.out.println("Enter the number of row to print");
        int rows = scanner.nextInt();
        
        int floydNumber=1;
        System.out.println("-- Printing --");
        for(int i=1;i<=rows;i++)
        {
            for(int j=1;j<=i;j++)
            {
                System.out.print("* ");
                floydNumber++;
            }
            System.out.println();
        }
    }
}

Output:

Enter the number of row to print
5
-- Printing --
* 
* * 
* * * 
* * * * 
* * * * *

Reverse Floyd’s Triangle in Java / Inverted  Triangle

package com.javainterviewpoint;

import java.util.Scanner;

/**
 * @author JavaInterviewPoint
 *
 */
public class ReverseFloydsTriangle
{
    /**
     * @param args
     */
    public static void main(String[] args)
    {
      //Create a new Scanner object
        Scanner scanner = new Scanner(System.in);
        
        //Get the number of rows from the user
        System.out.println("Enter the number of row to print");
        int rows = scanner.nextInt();
        
        int floydNumber = rows * (rows+1) / 2;
        
        System.out.println("** Printing **");
        for(int i=rows;i>=1;i--)
        {
            for(int j=1;j<=i;j++)
            {
                System.out.print(floydNumber+" ");
                floydNumber = floydNumber - 1;
            }
            System.out.println();
        }
    }
}

Output:

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

Floyd Triangle in Java using while loop

package com.javainterviewpoint;

import java.util.Scanner;

/**
 * @author JavaInterviewPoint
 *
 */
public class FloydTriangle
{
    
    /**
     * @param args
     */
    public static void main(String[] args)
    {
        //Create a new Scanner object
        Scanner scanner = new Scanner(System.in);
        
        //Get the number of rows from the user
        System.out.println("Enter the number of row to print");
        int rows = scanner.nextInt();
        
        int floydNumber=1;
        System.out.println("-- Printing --");
        
        int i =1;
        int j= 1;
        
        while (i <=rows)
        {
            j=1;
            while (j<=i)
            {
                System.out.print(floydNumber+" ");
                floydNumber++;
                j++;
            }
            System.out.println();
            i++;
        }
    }
}

Output:

Enter the number of row to print
7
-- Printing --
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