Recursive Solution To Sudoku Generator

I'm trying to code an algorithm that creates a legal Sudoku board in either Java or Javascript. Neither work, and I'm not entirely sure why. Essentially, the problem in both progr

Solution 1:

In the Java code: I'll translate it to psuedocode:

forall z values:
    If for current (x,y), the number 'z'is legal then:
        insert z to current (x,y)
        if finished
            hooray!
        else 
            go to next square
    elsetrynext number

But what if you can't put any number there as it ends up being illegal (aka a board where you can't insert any number in a specific square)?

You don't address that. What you need to do is implement it via backtracking:

forall z values:
    If forcurrent (x,y) the number 'z'is legal then:
        insert z tocurrent (x,y)
        go to next(x,y)
            try to complete the board    //recursivecall
        if you completed the board       //== the resultof the recursion is legal
            return the completed board
    If all z values have been attempted
        return "cannot complete board"
    increment z, try again withcurrent (x,y)

Solution 2:

Java:

1. You should initialize your board variable, you may want to initialize it in a constructor:

   publicclassSudokuGenerator {
   
       publicstaticfinalint BOARD_WIDTH = 9;
       publicstaticfinalint BOARD_HEIGHT = 9;
   
       publicSudokuGenerator(){
           board = newint[BOARD_WIDTH][BOARD_HEIGHT];
       }
   }
   

2. I believe that your loop iterator in the function nextBoard it is wrong:

   for(int i=1;i<10;i++){ ... }

   I think that you want to iterate from 0 to 9.

3. In the function nextBoard, you also need to check the variable:

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

   You get an java.lang.ArrayIndexOutOfBoundsException, you should initialize it from 0 to 8, otherwise you try to access the board row number 9 and you get a runtime error.

4. Another problem that you need to solve is that x is being set to nine in nextBoard() function. Call the function nextBoard(int x, int y) "manually" with these parameteres: nextBoard(7,3) and you will understand why x is being set to nine. Check specifically the values of the variable nextX.

I believe it will really help you if you use a debugger to check this kind of errors, here you have a nice tutorial with a video explanation(in case your are using the Eclipse IDE).

Hope it helps.

Solution 3:

Java:

Your loop iterator in nextBoard range from 1 to 9. I don't think you meant that. Same in the function legalMove.... initialize cornerX and cornerY to 0.

Solution 4:

interesting question, I just noticed this one bug in the Java code: isn't the call to Collection.shuffle() useless since the toCheck array will remain unmodifed (unshuffled) after this call? Here is my quick fix (and I'm sure there are more clever ways to do it):

List<Integer> lst = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9);
Collections.shuffle(lst);
for (int i=0; i<lst.size(); i++)
    toCheck[i] = lst.get(i);

Solution 5:

In Java array indexes are zero-based. In nextBoard you loop over 1..9 for i and use it as an index into toCheck which will skip the first element at index 0 and go past the end of the array. This will throw ArrayIndexOutOfBoundsException if the line containing toCheck[i] is reached with i equal to 9.