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Java Program to Print Left Triangle Number Pattern - 2

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In this program we print a left triangle number pattern where each row repeats its row number. The rows are lined up on the right side, so the triangle leans to the left.

Like every pattern program, it is built from nested loops: the outer loop picks the row, and the inner loops print what goes on that row.

1. The Pattern

For 5 rows the program prints:

Pattern
    1
   22
  333
 4444
55555

2. How the Program Works

Here is what each printed line is made of when numberOfRows is 5:

LineLeading spacesPrinted after the spaces
141
2322
32333
414444
5055555
  • The outer loop runs once for each row, from 1 to numberOfRows.
  • The first inner loop prints numberOfRows - row spaces.
  • The second inner loop prints row on each of its row passes.
  • System.out.println() moves to the next line after each row.

3. Java Program

Pattern5.java 
public class Pattern5 {
    public static void main(String[] args) {
        int numberOfRows = 5;

        // outer loop for the rows
        for (int row = 1; row <= numberOfRows; row++) {
            // inner loop to print the spaces
            for (int column = 1; column <= numberOfRows - row; column++) {
                System.out.print(" ");
            }
            // inner loop to print the row number, row times
            for (int column = 1; column <= row; column++) {
                System.out.print(row);
            }
            // move to the next line
            System.out.println();
        }
    }
}
Output
    1
   22
  333
 4444
55555

4. Print the Pattern for Any Number of Rows

To let the user choose the size of the pattern, read the number of rows with a Scanner. The loops stay the same.

Pattern5Input.java 
import java.util.Scanner;

public class Pattern5Input {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter the number of rows: ");
        int numberOfRows = scanner.nextInt();
        scanner.close();

        // outer loop for the rows
        for (int row = 1; row <= numberOfRows; row++) {
            // inner loop to print the spaces
            for (int column = 1; column <= numberOfRows - row; column++) {
                System.out.print(" ");
            }
            // inner loop to print the row number, row times
            for (int column = 1; column <= row; column++) {
                System.out.print(row);
            }
            // move to the next line
            System.out.println();
        }
    }
}
Output (with sample input)
Enter the number of rows: 6
     1
    22
   333
  4444
 55555
666666
Note: once the pattern needs numbers above 9, two-digit numbers take up two positions and the shape is no longer lined up. Print each number with a fixed width, for example System.out.printf("%3d", value), and print three spaces wherever the program prints one, to keep it lined up.

5. Summary

  • The outer loop picks the row; the inner loops print the spaces and the numbers for that row.
  • Work out how many spaces and which numbers each row needs, then write one inner loop for each.
  • Keep numberOfRows in a variable, so the same program can print the pattern in any size.

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