Java Program to Print Lower Triangle Matrix Including Principal Diagonal Matrix

This Java program efficiently prints the lower triangle matrix of a given square matrix, including the principal diagonal elements. It utilizes nested loops to traverse the matrix and selectively prints the lower triangle elements.

Program

package triangle;
import java.io.*;
import java.util.*;
public class PrintLowerTriangleMatrixPDE
{
	public static void main(String[] args) throws IOException
	{
		InputStreamReader isr = new InputStreamReader(System.in);
		BufferedReader br = new BufferedReader(isr);
		System.out.println("Enter the number of rows:");
		int row = Integer.parseInt(br.readLine());
		System.out.println("Enter the number of columns:");
		int col = Integer.parseInt(br.readLine());
		int mat[][] = new int [row][col];
		System.out.println("Enter " + (row * col) + " values into matrix");
		for(int i = 0; i < row;i++)
		{
			for(int j = 0; j < col; j++)
			{
				mat[i][j] = Integer.parseInt(br.readLine());
			}
		}
		System.out.println("Values in lower triangular matrix is:");
		for(int i = 0; i < row;i++)
		{
			for(int j = 0; j < col;j++)
			{
				if(i >= j)
					System.out.print(mat[i][j] + "\t");
				else
					System.out.print("*" + "\t");
			}
			System.out.println();
		}
	}
}

Let us understand the program by breaking into simple parts: An InputStreamReader is a bridge from byte streams to character streams: It reads bytes and decodes them into characters using a specified charset. It extends the abstract class Reader.

The above program prompts the user to enter the number of rows and columns for the matrix. It reads these values from the standard input (keyboard) using BufferedReader and readLine() method, and converts them into integers using Integer.parseInt(). 2D array mat[][] is created to store the matrix data, with dimensions determined by the user-input row and column values. Then the user is again prompted to input values for each cell of the matrix.

Nested for loops are used to take inputs from the user for each cell and save it in the form of array. After each cell value of the matrix is collected, the program uses the nested for loop and prints lower triangular part of the matrix excluding principal diagonal elements.

For each cell, it checks if the row index i is greater than or equal to the column index j. If so, it prints the value from the matrix; otherwise, it prints a "*" indicating it's not part of the lower triangular portion. The lower triangular part of the matrix consists of the elements where the row index is greater than or equal to the column index.

Output

Enter the number of rows:
3
Enter the number of columns:
3
Enter 9 values into matrix
52
41
67
30
12
28
97
81
36
Values in lower triangular matrix is:
52	*	*	
30	12	*	
97	81	36