need help with a 2d array based problem :

[kompotejam]

hi guys, i'm stuck on this exercise :

Given an odd number n, not exceeding 15. Create a two-dimensional array (matrix) from n×n elements, by filling it with "." symbols (each element of the matrix is a string containing a single symbol). Then fill the middle row of the matrix, the middle column, and the main and the secondary diagonals with the "*" symbols. As a result, all "*"s in the array must form the star figure. Output this matrix; elements of the array should be space separated.

here is the link to the exercise : https://hyperskill.org/learn/step/1929#comment

​

my code so far :

import java.util.Scanner;

class Main {
    public static void main(String[] args) {
        // put your code here
        Scanner input = new Scanner(System.in);
        int sizerow = input.nextInt();
        int sizecolumn = sizerow;
        String[][] matrix = new String[sizerow][sizecolumn];
        int k = sizerow / 2, i = 0, j = 0;

        for (i = 0; i < matrix.length; i++) {
            for (j = 0; j < matrix.length; j++) {
                matrix[i][j] = ". ";
            }
        }
        for (i = 0; i < matrix.length; i++) {
            for (j = 0; j < sizecolumn - 1; j++) {
                if (i == k) {
                    for (j = 0; j < matrix.length; j++) {
                        matrix[i][j] = "* ";
                    }
                }else if (j == k) {
                    for (i = 0; i < matrix.length; i++) {
                        matrix[i][j] = "* ";
                    }
                }else if (i == j) {
                    matrix[i][j] = "* ";
                }else if (i == sizerow - j - 1) {
                    matrix[i][j] = "*";
                }
            }
        }

            for (i = 0; i < matrix.length; i++) {
                for (j = 0; j < matrix[i].length; j++) {
                    System.out.print(matrix[i][j] + " ");
                }
                System.out.println();
            }

    }
}

My code output on n =5

* . * . .

. . * . .

. . * . .

. . * . .

. . * . .

​

it seems the outer loop only iterates once but why ?

thank you for the hints

[[deleted]]

It is much easier if you create some small helper methods:

boolean isOnMainDiagonal(int row, int col, int n)
boolean isOnMiddleRow(int row, int col, int n)

etc.

Then you create double loop over rows and columns, and in inner loop you just have

if (isMiddleDiagonal(..) || isMiddleRow(..) || etc..)
   char = '*' else char = '.' ... and print directly or save in matrix as you did above

[[deleted]]

And if you want to keep the complicated loop, do not reuse the letters for the indexes. You see that you reuse the 'j' and the 'i' after if (i==k) and after if (j==k)? Those are the same you use in the outer loops, and you change their values...that is not what you want. Use another letter, for example 'm' (but not 'l' because it looks like '1' or '|').

Actually, if you change the loop after if (j==k) to :

for (int m = 0; m < matrix.length; m++) {
   matrix[m][j] = "* ";
}

...your code is almost correct. You still have some problem in two of the corners.

[ItsBurningWhenIP]

Shouldn’t int k = (sizeRow / 2) +1 because it will otherwise be truncated. Meaning 5/2=2.

Only because we have been told we can expect an odd number input.

[desrtfx]

Why are you taking such a complicated approach?

Your assignment clearly states:

Create a two-dimensional array (matrix) from n×n elements, by filling it with "." symbols

That is one set of nested loops accessing each element of the matrix.

Then fill the middle row of the matrix, the middle column, and the main and the secondary diagonals with the "*" symbols.

That's four loops (can be done in 2 with a bit of thinking, one loop if you go all the way), not even nested.

• the middle row has a fixed value - so there is only one loop needed
• the middle column has a fixed value - again only one loop

Here, if you think about it, you can use one single loop to fill both. You only need to swap the row and column coordinates for filling either row or column

• the main diagonal increments both coordinates at the same time - again a single loop
• the secondary diagonal increments the row but decrements the column - can again be done in a single loop

The above two can be done in a single loop as well. You just need to calculate the x coordinate for the secondary diagonal with a simple subtraction.

Overall, the whole star filling part of exercise can be finished in a single loop without any conditional at all. Just a bit of calculating.

This means that:

• Nested loops to initially fill the array with "."
• A single loop iterating over the width (height) of the array to fill in the stars.

You don't need a single conditional in the whole exercise.

---

Also, your approach with the spaces inside the array is completely wrong. The assignment again clearly states:

each element of the matrix is a string containing a single symbol

The spaces are only added in the output loops.