Question

I’ve asked earlier a question about solving the eight queens problem using Java.
I got a backtracking algorithm to solve the problem.

I tried to use this algorithm but I don’t know what’s wrong with my code. It places up to 7 queens only.

Here’s Queen Class:

    public class Queen {
    //Number of rows or columns
    public static final int BOARD_SIZE = 8;

    boolean[][] board;
    //Indicate an empty square
    public static final boolean EMPTY = false;
    //Indicate a square which containing a queen
    public static final boolean QUEEN = true;
    //Number of moves
    public static final int MOVES = 4;
    //Horizontal moves
    int[] horizontal;
    //Vertical moves
    int[] vertical;

    public int queens = 0;

    public Queen() {
        //Constructor creates an empty board
        board = new boolean[BOARD_SIZE][BOARD_SIZE];
        for (int row = 0; row < board.length; row++) {
            for (int col = 0; col < board[row].length; col++) {
                board[row][col] = EMPTY;
            }
        }

        horizontal = new int[MOVES];
        vertical = new int[MOVES];
        // up right
        horizontal[0] = -1;
        vertical[0] = 1; 
        // down left
        horizontal[1] = 1;
        vertical[1] = -1;
        // up left
        horizontal[2] = -1;
        vertical[2] = -1;
        // down right
        horizontal[3] = 1;
        vertical[3] = 1;
    }

    public boolean placeQueens (int column) {
        if (column > BOARD_SIZE) {
            return true;
        }
        else {
            boolean queenPlaced = false;
            int row = 1;

            while (!queenPlaced && row < BOARD_SIZE) {
                if (isUnderAttack(row, column)) {
                    ++row;
                }// end if
                else{
                    setQueen(row, column);
                    queenPlaced = placeQueens(column + 1);
                    if (!queenPlaced) {
                        removeQueen(row,column);
                        ++row;
                    }// end if
                }// end else
            }// end while
            return queenPlaced;
        }// end else
    }

    private void removeQueen(int row, int column) {
        board[row][column] = EMPTY;
        System.out.printf("queen REMOVED from [%d][%d]\n", row, column);
    --queens;
    }

    private void setQueen(int row, int column) {
        board[row][column] = QUEEN;
        System.out.printf("queen PLACED in [%d][%d]\n", row, column);
        ++queens;
    }

    public boolean isUnderAttack(int row, int col) {
        boolean condition = false;
        // check row
        for (int column = 0; column < BOARD_SIZE; column++) {
            if ((board[row][column] == true)) {
                condition = true;
            }
        }

        // check column
        for (int row_ = 0; row_ < board.length; row_++) {
            if (board[row_][col] == true) {
                        condition = true;
            }
        }

        // check diagonal
        for (int row_ = row, col_ = col; row_ >= 0 && col_ < 8; row_ += horizontal[0], col_ += vertical[0]) {
            if (board[row_][col_] == true) {
                condition = true;
            }
        }
        for (int row_ = row, col_ = col; row_ < 8 && col_ >= 0; row_ += horizontal[1], col_ += vertical[1]) {
            if (board[row_][col_] == true) {
                condition = true;
            }
        }
        for (int row_ = row, col_ = col; row_ >= 0 && col_ >= 0; row_ += horizontal[2], col_ += vertical[2]) {
            if (board[row_][col_] == true) {
                condition = true;
            }
        }
        for (int row_ = row, col_ = col; row_ < 8 && col_ < 8; row_ += horizontal[3], col_ += vertical[3]) {
            if (board[row_][col_] == true) {
                condition = true;
            }
        }

        return condition;
    }

    public void displayBoard () {
        int counter = 0;
        for (int row = 0; row < board.length; row++) {
            for (int col = 0; col < board[row].length; col++) {
                if (board[row][col] == true) {
                    System.out.printf("|%s|", "x");
                    counter++;
                }
                else {              
                    System.out.printf("|%s|", "o");
                }
            }
            System.out.println();
        }

        System.out.printf("%d queens has been placed\n", counter);
    }
}

Answer 1

In Java arrays are 0-indexed, meaning that the first item is at index 0. You seem to have not entirely grasped this crucial fact, which has resulted in you writing code with many off-by-one errors.

One problem is here:

int row = 1;

It should be:

int row = 0;

---

A second problem is here:

if (column > BOARD_SIZE) {
    return true;
}

It should be this:

if (column >= BOARD_SIZE) {
    return true;
}

---

You haven’t posted the rest of your code, but I’m willing to bet that there is a third error in your code when you call the placeQueens method. If I know what sort of person you are, then you probably did this:

queen.placeQueens(1);

But it should be this:

queen.placeQueens(0);

---

With all these changes it works as expected. The final result is:

|x||o||o||o||o||o||o||o|
|o||o||o||o||o||o||x||o|
|o||o||o||o||x||o||o||o|
|o||o||o||o||o||o||o||x|
|o||x||o||o||o||o||o||o|
|o||o||o||x||o||o||o||o|
|o||o||o||o||o||x||o||o|
|o||o||x||o||o||o||o||o|
8 queens has been placed

See it working online: ideone