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Java Program to Print Left Triangle Star Pattern

Last updated: 2 min read

In this program we print a left triangle of stars. The rows are lined up on the right side: each row has one more star than the row above it, and spaces in front push it to the right.

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
        *
      * *
    * * *
  * * * *
* * * * *

2. How the Program Works

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

LineLeading spacesPrinted after the spaces
18*
26* *
34* * *
42* * * *
50* * * * *
  • The outer loop runs once for each row, from 1 to numberOfRows.
  • The first inner loop prints 2 * (numberOfRows - row) spaces. Each star is printed as "* " and takes two positions, so every missing star needs two spaces.
  • The second inner loop prints row stars.
  • System.out.println() moves to the next line after each row.

3. Java Program

LeftTriangleStarPattern.java 
public class LeftTriangleStarPattern {
    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, two for each missing star
            for (int column = 1; column <= 2 * (numberOfRows - row); column++) {
                System.out.print(" ");
            }
            // inner loop to print the stars
            for (int column = 1; column <= row; column++) {
                System.out.print("* ");
            }
            // move to the next line
            System.out.println();
        }
    }
}
Output
        *
      * *
    * * *
  * * * *
* * * * *

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.

LeftTriangleStarPatternInput.java 
import java.util.Scanner;

public class LeftTriangleStarPatternInput {
    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, two for each missing star
            for (int column = 1; column <= 2 * (numberOfRows - row); column++) {
                System.out.print(" ");
            }
            // inner loop to print the stars
            for (int column = 1; column <= row; column++) {
                System.out.print("* ");
            }
            // move to the next line
            System.out.println();
        }
    }
}
Output (with sample input)
Enter the number of rows: 6
          *
        * *
      * * *
    * * * *
  * * * * *
* * * * * *

5. Summary

  • The outer loop picks the row; the inner loops print the spaces and the stars for that row.
  • Work out how many spaces and stars 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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