Java array out of bounds - sudoku

Question

My code for filling sudoku board looks like this:

public class SudokuBoard {

    static int N = 9;
    static int[][] grid = new int[N][N];

    static void printGrid()
    {
        for (int row = 0; row < N; row++)
        {
            for (int col = 0; col < N; col++) {
                System.out.printf("%5d", grid[row][col]);
            }
            System.out.println("\n");
        }
    }

    private static boolean checkRow(int row, int num)
    {
        for( int col = 0; col < 9; col++ )
            if(grid[row][col] == num)
                return false;

        return true;
    }

    private static boolean checkCol(int col, int num)
    {
        for( int row = 0; row < 9; row++ )
            if(grid[row][col] == num)
                return false;

        return true;
    }

    private static boolean checkBox(int row, int col, int num)
    {
        row = (row / 3) * 3;
        col = (col / 3) * 3;

        for(int r = 0; r < 3; r++)
            for(int c = 0; c < 3; c++)
                if(grid[row+r][col+c] == num)
                    return false;

        return true;
    }

    public static boolean fillBoard(int row, int col, int[][] grid)
    {
        if(row==9)
        {
            col = 0;
            if(col++ == 9)
                return true;
        }
        if(grid[row][col] != 0)
            return fillBoard(row+1, col, grid);
        for(int num = 1; num <=9; num++)
        {
            if(checkRow(row,num) && checkCol(col,num) && checkBox(row,col,num)){
                grid[row][col] = num;
                if(fillBoard(row+1, col, grid))
                    return true;
            }
        }
        grid[row][col] = 0;
        return false;
    }

    static public void main(String[] args){
        fillBoard(0, 0, grid);
        printGrid();
    }
}

It uses backtrack algorithm to check if placement of numbers are good according to sudoku game puzzle rules. It throws errors:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 9
at SudokuBoard.fillBoard(SudokuBoard.java:68)
at SudokuBoard.fillBoard(SudokuBoard.java:74) x9
at SudokuBoard.main(SudokuBoard.java:84)

Where is it out of bounds? I cannot see that...

Doesn't have to go over 9. Anything over 8 will throw the exception. — Luka Jacobowitz, Mar 15 '16 at 23:09
but how even it goes over 8? if rows and columns are limited — szubansky, Mar 15 '16 at 23:11

Paul Ostrowski · Accepted Answer · 2016-03-15T23:18:42.797

1

This block looks wrong:

    if(row==9)
    {
        col = 0;
        if(col++ == 9)
            return true;
    }

I suspect you want this:

    if(row==9) {
        row = 0;
        if(++col == 9)
            return true;
     }

edited Mar 15 '16 at 23:18

answered Mar 15 '16 at 23:10

Paul Ostrowski

1,968
16
21

score 0 · Answer 2 · edited May 23 '17 at 11:59

col++ increments col, but returns the old value. You probably meant to use ++col, which returns the new value. (see Java: Prefix/postfix of increment/decrement operators?)

In your code, when fillBoard(8, 8, grid) is called, col is increased to 9, but (col++ == 9) gets evaluated to false, because col++ returns 8. So you then try to access grid[8][9], which is when the exception is thrown.

Java array out of bounds - sudoku

2 Answers2