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Java Program to Print Left Triangle Number Pattern - 1

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In this program we print a left triangle number pattern. The numbers are lined up on the right side: each row counts from 1 up to the row number, 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.

1. The Pattern

For 5 rows the program prints:

Pattern
    1
   12
  123
 1234
12345

2. How the Program Works

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

LineLeading spacesPrinted after the spaces
141
2312
32123
411234
5012345
  • 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 the numbers from 1 to row.
  • 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 numbers 1 to row
            for (int column = 1; column <= row; column++) {
                System.out.print(column);
            }
            // move to the next line
            System.out.println();
        }
    }
}
Output
    1
   12
  123
 1234
12345

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 numbers 1 to row
            for (int column = 1; column <= row; column++) {
                System.out.print(column);
            }
            // move to the next line
            System.out.println();
        }
    }
}
Output (with sample input)
Enter the number of rows: 6
     1
    12
   123
  1234
 12345
123456
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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