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Java Program to Print Right Triangle Outline Star Pattern

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In this program we print the outline of a right triangle. Each row prints a star at its first and last position, and the last row is a full line of stars.

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
10*
20* *
30*   *
40*     *
50* * * * *
  • The outer loop runs once for each row, from 1 to numberOfRows.
  • The inner loop runs row times. It prints "* " in the first column, in the last column and on every column of the last row.
  • Every other position prints two spaces, the same width as "* ", so the edges stay lined up.
  • System.out.println() moves to the next line after each row.

3. Java Program

RightTriangleOutlineStarPattern.java 
public class RightTriangleOutlineStarPattern {
    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 outline: the edges, then the whole last row
            for (int column = 1; column <= row; column++) {
                if (row == numberOfRows || column == 1 || column == row) {
                    System.out.print("* ");
                } else {
                    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.

RightTriangleOutlineStarPatternInput.java 
import java.util.Scanner;

public class RightTriangleOutlineStarPatternInput {
    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 outline: the edges, then the whole last row
            for (int column = 1; column <= row; column++) {
                if (row == numberOfRows || column == 1 || column == row) {
                    System.out.print("* ");
                } else {
                    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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