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Java Program to Print Double Triangle Number Pattern - 2

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In this program we print two right triangles one above the other: the first shrinks from 1 2 3 4 5 down to 1, and the second grows back from 1 to 1 2 3 4 5.

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 2 3 4 5
1 2 3 4
1 2 3
1 2
1
1
1 2
1 2 3
1 2 3 4
1 2 3 4 5

2. How the Program Works

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

LineLeading spacesPrinted after the spaces
101 2 3 4 5
201 2 3 4
301 2 3
401 2
501
601
701 2
801 2 3
901 2 3 4
1001 2 3 4 5
  • Both parts print the numbers from 1 to row on each row.
  • Step 1 counts row down from numberOfRows to 1, which prints the downward triangle.
  • Step 2 counts row up from 1 to numberOfRows, which prints the upward triangle. Both parts print a row with only 1, so 1 appears twice in the middle.
  • System.out.println() moves to the next line after each row.

3. Java Program

Pattern19.java 
public class Pattern19 {
    public static void main(String[] args) {
        int numberOfRows = 5;

        // Step 1: downward triangle, from the longest row to the shortest
        for (int row = numberOfRows; row >= 1; row--) {
            for (int column = 1; column <= row; column++) {
                System.out.print(column + " ");
            }
            System.out.println();
        }

        // Step 2: upward triangle, from the shortest row to the longest
        for (int row = 1; row <= numberOfRows; row++) {
            for (int column = 1; column <= row; column++) {
                System.out.print(column + " ");
            }
            System.out.println();
        }
    }
}
Output
1 2 3 4 5
1 2 3 4
1 2 3
1 2
1
1
1 2
1 2 3
1 2 3 4
1 2 3 4 5

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.

Pattern19Input.java 
import java.util.Scanner;

public class Pattern19Input {
    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();

        // Step 1: downward triangle, from the longest row to the shortest
        for (int row = numberOfRows; row >= 1; row--) {
            for (int column = 1; column <= row; column++) {
                System.out.print(column + " ");
            }
            System.out.println();
        }

        // Step 2: upward triangle, from the shortest row to the longest
        for (int row = 1; row <= numberOfRows; row++) {
            for (int column = 1; column <= row; column++) {
                System.out.print(column + " ");
            }
            System.out.println();
        }
    }
}
Output (with sample input)
Enter the number of rows: 4
1 2 3 4
1 2 3
1 2
1
1
1 2
1 2 3
1 2 3 4

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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