Randomly populating a 2d array

Question

I am using Java and working on 5X5 board game(represented as String[][]) and I looking for an efficient way to randomly place 3 "A"'s, 3 "B",s 3 "C"'s, 3 "D"'s on the board.

I thought about using a nested for loop inside of a while loop to go over each slot, and randomly assign letters to 'slots' on the board, but I want to possibly do it in one pass of the board, if there is a good way to randomly place all 15 letters on the board in one pass.

Any suggestions?

Answer 1

One way: create an ArrayList<Character> or of String, feed it the 15 letters, call java.util.Collections.shuffle(...) on the ArrayList, and then iterate over this List, placing its randomized items into your array.

eg,

  List<String> stringList = new ArrayList<String>();
  for (char c = 'A'; c <= 'E'; c++) {
     for (int i = 0; i < 3; i++) {
        stringList.add(String.valueOf(c));
     }
  }
  for (int i = 15; i < 25; i++) {
     stringList.add(null);
  }

  Collections.shuffle(stringList);
  String[][] gameBoard = new String[5][5];
  for (int i = 0; i < gameBoard.length; i++) {
     for (int j = 0; j < gameBoard[i].length; j++) {
        gameBoard[i][j] = stringList.get(i * gameBoard.length + j);
     }
  }

  // now test it
  for (int i = 0; i < gameBoard.length; i++) {
     for (int j = 0; j < gameBoard[i].length; j++) {
        System.out.printf("%-6s ", gameBoard[i][j]);
     }
     System.out.println();
  }

Answer 2

You can use an ArrayList to store the letters (and the empty cells, I'm using a dot . so you can recognize it in the output), then use Collections.shuffle() to put elements of the ArrayList in "random" places. Finally assign each letter to the String[][] array:

public static void main(String[] args)
{
    String[][] board = new String[5][5];
    List<String> letters = new ArrayList<>();

    // fill with letters
    for (int i = 0; i < 3; i++) {
        letters.add("A");
        letters.add("B");
        letters.add("C");
        letters.add("D");
        letters.add("E");
    }
    // fill with "empty"
    for (int i = 0; i < 10; i++) {
        letters.add(".");
    }

    Collections.shuffle(letters);

    for (int i = 0; i < board.length; i++) {
        for (int j = 0; j < board.length; j++) {
            board[i][j] = letters.get(i*board.length + j);
        }
    }

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

Output Sample:

C B . . . 
A E . E A 
. A . D D 
C . . . D 
B C E . B 

Note:

The operation i*board.length + j will generate consequent numbers 0, 1, 2, 3, ... 24 en the nested loop.