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Java Program to Print Left Triangle Character Pattern - 1

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In this program we print a left triangle of letters. The rows are lined up on the right side: each row starts at A, and spaces in front of it 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. 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
   AB
  ABC
 ABCD
ABCDE

2. How the Program Works

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

LineLeading spacesPrinted after the spaces
14A
23AB
32ABC
41ABCD
50ABCDE
  • 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 letters, starting at A.
  • System.out.println() moves to the next line after each row.

3. Java Program

Pattern4.java 
public class Pattern4 {
    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 letters A, B, C ... (row letters)
            for (int column = 1; column <= row; column++) {
                System.out.print((char) ('A' + column - 1));
            }
            // move to the next line
            System.out.println();
        }
    }
}
Output
    A
   AB
  ABC
 ABCD
ABCDE

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.

Pattern4Input.java 
import java.util.Scanner;

public class Pattern4Input {
    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 letters A, B, C ... (row letters)
            for (int column = 1; column <= row; column++) {
                System.out.print((char) ('A' + column - 1));
            }
            // move to the next line
            System.out.println();
        }
    }
}
Output (with sample input)
Enter the number of rows: 6
     A
    AB
   ABC
  ABCD
 ABCDE
ABCDEF
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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