'2D' Array Sudoku Puzzle Check with enum. Cannot understand the flow of my own program despite debugging

Question

I am programming a simple (or so I thought!) Checker for Sudoku puzzles using a '2D' array and an enum. The program successfully checks the grid and reports "Solution Okay!" - but it has a strange 'side effect - it prints out "invalid ROW 4" and "invalid COLUMN 1". I am new to programming and despite best efforts debugging I cannot find a solution.

(This problem was taken from CS106A Stanford University course- there are suggested solutions but I want to understand the flaw in my own code. I know it's a dirty solution and perhaps not a good example of programming. All feedback welcome.)

ROWS 1 Vaild
ROWS 2 Vaild
ROWS 3 Vaild
ROWS invalid! number: 4
ROWS 4 Vaild
ROWS 5 Vaild
ROWS 6 Vaild
ROWS 7 Vaild
ROWS 8 Vaild
ROWS 9 Vaild
All ROWS Valid
COLUMNS invalid! number: 1
COLUMNS 1 Vaild
COLUMNS 2 Vaild
COLUMNS 3 Vaild
COLUMNS 4 Vaild
COLUMNS 5 Vaild
COLUMNS 6 Vaild
COLUMNS 7 Vaild
COLUMNS 8 Vaild
COLUMNS 9 Vaild
All COLUMNS Valid
Solution Okay! 

Separate file but same package

public enum Direction{
    ROWS,COLUMNS,SQUARES
}

//end of enum

//import java.util.zip.Checksum;

import acm.program.ConsoleProgram;

/**
 * Program that receives in a grid and checks that it is a valid sudoku solution
 * 
 *
 */


public class SudokuTest extends ConsoleProgram  {


static boolean result;
Direction direction;

    public static void main(String args[]){

        int[][] grid = new int[][]{
                {2,5,7,9,6,4,1,8,3},
                {4,9,1,8,7,3,6,5,2},
                {3,8,6,1,2,5,9,4,7},
                {6,4,5,7,3,2,8,1,9},
                {7,1,9,5,4,8,3,2,6},
                {8,3,2,6,1,9,5,7,4},
                {1,6,3,2,5,7,4,9,8},
                {5,7,8,4,9,6,2,3,1},
                {9,2,4,3,8,1,7,6,5}
                };

        SudokuTest st = new SudokuTest();
        result = st.checkSudokuSolution(grid);
        System.out.println("The solution is correct: " + result);


    }
    private boolean checkSudokuSolution(int[][] grid){

        //test works with line below

        boolean allRowsOkay = checkElement(grid,direction.ROWS);
        boolean allCOlumnsOkay = checkElement(grid,direction.COLUMNS);

        if(allCOlumnsOkay&allCOlumnsOkay==true){
        System.out.println("Solution Okay! ");
        return true;
    }
    else
        return false;
}

    private boolean checkElement(int[][] grid,Direction direction) {
        boolean []hist =new boolean[10];
        int value=0; //value of the current entry
        int elementCount=0;
        int count=1;
        int i;
        int j;

        //program using histogram type solution
        //Check rows first - need to consider whether code can be reused for columns

        for(i=0;i<=8;i++){
            for(j=0;j<=8;j++){

        if(direction == direction.ROWS)     
                {
            value = grid[i][j];
            }


        else if(direction == direction.COLUMNS) {
            value = grid[j][i];
            }
        else {

            break;
        }

                hist[value] = true;


        //can i reuse code for row and column?
            if(hist[1]&hist[2]&hist[3]&hist[4]&hist[5]&hist[6]&hist[7]&hist[8]&hist[9]==true) {

                System.out.println("" + direction +" "+(i+1) + " Vaild");
                elementCount++;
                hist = new boolean[10];
                }

            else if(hist[1]&hist[2]&hist[3]&hist[4]&hist[5]&hist[6]&hist[7]&hist[8]&hist[9]==false)  {

                System.out.println(direction + " invalid! " + "number: " + (i+1));

            }

            }
            hist = new boolean[10];
        }

        if(elementCount==9)System.out.println("All " + direction + " Valid");



        if(elementCount==9)return true;
        else return false;

        }
    }

Answer 1

The statements checking the 'hits' array are very poorly formatted. the & operator is a bitwise AND. I expect you mean this to be '&&` which is logical AND. Also, in general, you don't need to compare a boolean value to true or false.

So your comparisons should read something like:

if ( hist[1] && hist[2] && ... && hist[9]) {
    ...
} else {
    ...
}

Secondly, you are running these checks 9 times per row or column (I think that's right - the indenting is mixed up so it's hard to tell). You should be running these checks outside your j for loop. In other words, it should build the hist array completely for a row or column and then check whether all 9 values are accounted for.

There are many other stylistic errors and possible logic errors but these seem to me to be the first things to fix.

Answer 2

Sorry, but if you can't understand your own code, it's time to re-write it. If this were my project, I'd try to divide and conquer by:

• I'd fill the value array just as you're doing, within two nested for loops
• But that's all I'd do inside of those loops. This will keep my code simpler and make it easier to understand and to solve the problem.
• I'd analyze the value array after adding all the data to it, meaning after both nested for loops have concluded, not as you're doing it inside of the for loops.
• I would create a separate method to analyze a single row/column or 9 value square
• I'd have my checkElement method call these guys in a for loop.