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Java Program to Print Double Triangle Character Pattern

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In this program we print two letter triangles that meet at a point: the first shrinks from A B C D E down to A, and the second grows back out to A B C D E.

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. Java stores a char as a number, so adding to 'A' and casting back with (char) gives the following letters.

1. The Pattern

For 5 rows the program prints:

Pattern
A B C D E
A B C D
A B C
A B
A
A B
A B C
A B C D
A B C D E

2. How the Program Works

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

LineLeading spacesPrinted after the spaces
10A B C D E
20A B C D
30A B C
40A B
50A
60A B
70A B C
80A B C D
90A B C D E
  • Both parts print row letters, starting at A, each followed by a space.
  • Step 1 counts row down from numberOfRows to 1.
  • Step 2 counts row up from 2 to numberOfRows. It starts at 2 so the row with a single A is printed only once.
  • System.out.println() moves to the next line after each row.

3. Java Program

Pattern18.java 
public class Pattern18 {
    public static void main(String[] args) {
        int numberOfRows = 5;

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

        // Step 2: upward triangle, starting at A B so the single A is not repeated
        for (int row = 2; row <= numberOfRows; row++) {
            for (int column = 1; column <= row; column++) {
                System.out.print((char) ('A' + column - 1) + " ");
            }
            System.out.println();
        }
    }
}
Output
A B C D E
A B C D
A B C
A B
A
A B
A B C
A B C D
A B C D E

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.

Pattern18Input.java 
import java.util.Scanner;

public class Pattern18Input {
    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 a single A
        for (int row = numberOfRows; row >= 1; row--) {
            for (int column = 1; column <= row; column++) {
                System.out.print((char) ('A' + column - 1) + " ");
            }
            System.out.println();
        }

        // Step 2: upward triangle, starting at A B so the single A is not repeated
        for (int row = 2; row <= numberOfRows; row++) {
            for (int column = 1; column <= row; column++) {
                System.out.print((char) ('A' + column - 1) + " ");
            }
            System.out.println();
        }
    }
}
Output (with sample input)
Enter the number of rows: 4
A B C D
A B C
A B
A
A B
A B C
A B C D
Note: there are only 26 letters. Past Z, (char) ('A' + 26) is [, not a letter, so keep the number of rows small enough that the pattern never needs a letter after Z.

5. Summary

  • The outer loop picks the row; the inner loops print the spaces and the letters for that row.
  • (char) ('A' + k) gives the letter k places after A.
  • Keep numberOfRows in a variable, so the same program can print the pattern in any size.

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