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Java Program to Print Downward Left Triangle Number Pattern - 1

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In this program we print a downward left triangle number pattern. Every row counts from 1, each row is one number shorter than the row above it, and one more space in front keeps the right edge straight.

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
12345
 1234
  123
   12
    1

2. How the Program Works

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

LineLeading spacesPrinted after the spaces
1012345
211234
32123
4312
541
  • The outer loop runs once for each row, from 1 to numberOfRows.
  • The first inner loop prints row - 1 spaces.
  • The second inner loop prints the numbers from 1 to numberOfRows - row + 1.
  • System.out.println() moves to the next line after each row.

3. Java Program

Pattern10.java 
public class Pattern10 {
    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 < row; column++) {
                System.out.print(" ");
            }
            // inner loop to print the numbers 1 to (numberOfRows - row + 1)
            for (int column = 1; column <= numberOfRows - row + 1; column++) {
                System.out.print(column);
            }
            // move to the next line
            System.out.println();
        }
    }
}
Output
12345
 1234
  123
   12
    1

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.

Pattern10Input.java 
import java.util.Scanner;

public class Pattern10Input {
    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 < row; column++) {
                System.out.print(" ");
            }
            // inner loop to print the numbers 1 to (numberOfRows - row + 1)
            for (int column = 1; column <= numberOfRows - row + 1; column++) {
                System.out.print(column);
            }
            // move to the next line
            System.out.println();
        }
    }
}
Output (with sample input)
Enter the number of rows: 6
123456
 12345
  1234
   123
    12
     1
Note: once the pattern needs numbers above 9, two-digit numbers take up two positions and the shape is no longer lined up. Print each number with a fixed width, for example System.out.printf("%3d", value), and print three spaces wherever the program prints one, to keep it lined up.

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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