在随机位置用固定数量的宝物填充2D字符数组

Question

So, right now I have a 2D array, that prints a game field depending on user input(row and col). It fill the array with '.' chars. What I need now, is to use third user input amountTreasure to fixate the amount of treasures on the map.

How can I loop through this 2D array and put for example 3 treasures in random locations. The interesting part is that I need to prevent computer to randomly select the same place more than once.

I have this code right now.

public static char[][] createMatrix(int n, int m, int amountTreasure) {


    Random rand = new Random();
        char[][] matrix = new char[n][m];
        for (char[] matrixList : matrix) {
            Arrays.fill(matrixList, '.');
        }
        for (int v = 0; v < matrix.length; v++) { //Loop through matrix
            for (int b = 0; b < matrix[v].length; b++) {
                continue;
            }
        }
        return matrix;
    }

I tried something like

matrix[v][b] = (char) rand.nextInt('X')

but it does not work. I am really new to Java and do not know how to do that.

Answer 1

Instead of looping through the array, compute random locations and put the treasure there.

for(int tresasure = 0; treasure < amountTreasure; treasure++) {
    int x, y;
    do {
        x = random.nextInt(matrix.length);
        y = random.nextInt(matrix[x].length);
    } while(matrix[x][y] == 'X');
    matrix[x][y] = 'X';
}

Answer 2

Instead of looping through the array, have your Random return the coordinates of where a treasure should go. Then you just need to check whether by accident the same coordinates had been generated before.

Random random = new Random();

for (int i = 0; i < amountTreasure; i++) {
    int treasureX, treasureY;

    do {
        treasureX = random.nextInt(n);
        treasureY = random.nextInt(m);
    } while (matrix[treasureX][treasureY] == 'X');

    matrix[treasureX][treasureY] = 'X';
}

Answer 3

Here is one way of doing it by using the HashSet to prevent duplicates. It does not loop through the matrix to choose the random locations.

This is the code snippet:

public static char[][] createMatrix(int n, int m, int amountTreasure) {
    Random rand = new Random();
    char[][] matrix = new char[n][m];
    for (char[] matrixList : matrix) {
        Arrays.fill(matrixList, '.');
    }

    Set<String> hashSet = new HashSet<>();
    /* Select At Random */
    for(int iter = 0; iter < amountTreasure; iter++) {
        String trs = null;
        int randRow = -1;
        int randCol = -1;
        /* Generate New Random */
        while(!hashSet.contains(trs) && trs == null) {
            randRow = rand.nextInt(n);
            randCol = rand.nextInt(m);
            trs = new String(String.valueOf(n) + "," + String.valueOf(m));
        }
        /* Add In HashSet */
        hashSet.add(trs);
        matrix[randRow][randCol] = 'X';
    }
    /* Return Matrix */
    return matrix;
}

Output:

. . . . 
. . X . 
X . X . 
. . . .

Answer 4

You can look through your 2D-array and save the "empty" cells positions in another list and then choose randomly from them. That way you can't select one cell multiple times.

How to save cell positions? You can make extra class for cells:

class Cell {
      int x, y;
      public Cell(int x, y) {
             this.x = x;
             this.y = y;
      }
}

Then make ArrayList of Cells:

List<Cell> emptyCells = new ArrayList<Cell>();

Look through your 2D array and add empty cells there:

for (int v = 0; v < matrix.length; v++) { //Loop through matrix
    for (int b = 0; b < matrix[v].length; b++) {
        if(matrix[v][b] == '.') emptyCells.add(new Cell(v, b));
    }
}

And now you can randomly select from those.