Question

I am working a sudoku solver and am having trouble returning / ending the solver function correctly. The show() in the moveOn() function gets called and it displays the compleated sudoku fine, however solve returns false. I am trying to have solve return true when the problem is solved and null when it is unsolveable however do not know how to accomplish this.

L is the length of the board (a 9 x 9 sudoku would have L = 9)

getSquare(r,c) returns the value in a 2 dimensional array representing the sudoku board

the different check functions check to see if a value can fit in a specific location. They are not the issue.

the show() function prints out the array in console so it looks like a proper sudoku board.

I also have an isSolved() function that checks the 2D array and if it is a valid solved sudoku returns true, otherweise returns false. I have attempted to implement this as well into the solve() method hoping to use that to return true, though have had no success

//This method's only purpose it to call the findNum function on the next location in the sudoku
public void moveOn(int row, int column) {
    //if the previous location was not the last in the row move to ne next cell in said row.
    //if it was the last location in the row, move to the first column of the next row
    if (column + 1 != L) {solve(row, column + 1);}
    else if (row + 1 != L) {solve(row + 1, 0);}
    else {show();}
}

//This method finds a valid number for a specific location on the sudoku grid\
public boolean solve(int row, int column) {
    if (row >= L) {return true;}
    //pass over any numbers that are not empty
    if (getSquare(row, column) != 0) {moveOn(row, column);}
    else {
        //attempt to find a valid number for the location
        for (int n = 1; n <= L; n++) {
            if (checkRow(row, n) && checkCol(column, n) && checkSquare(row, column, n)) {
                // If a number is allowed at a specific location set that location to the number
                setSquare(row, column, n);
                //Begin checking for a solution based on previous numbers changed           
                moveOn(row, column);
            }               
        }
        //If no number is allowed in this space backtrack to the last successful number 
        //changed and reset all locations that have been changed recursively
        setSquare(row, column, 0);          
    }
    //If the puzzle is unsolveable
    return false;
}

Many thanks to anybody that can help shed some light on the situation.
If more of my code / information is needed I will gladly provide

Sample input file: http://pastebin.com/6mSKT3ES

Edit: complete code removed

Answer 1

You have only one return statement in the solve function, and that is

return false;

and since that is the last statement in the function, and unconditionally executed, solve will, unless an exception is thrown, always return false.

To get a return value that actually tells you whether you found a solution, you need to make the return value depend on a condition. Also, once you have found a solution, for well-posed puzzles, there is no point in continuing to search.

So you should add a conditional return true; in the searching loop. For that, you need to know when you have found a solution. You wrap the recursion in an intermediate call to moveOn, so the simplest change would be to add a return value to moveOn:

public boolean moveOn(int row, int column) {
    //if the previous location was not the last in the row move to ne next cell in said row.
    //if it was the last location in the row, move to the first column of the next row
    if (column + 1 != L) {return solve(row, column + 1);}
    else if (row + 1 != L) {return solve(row + 1, 0);}
    else {show(); return true;}  // reached end of grid, solved
}

and use that in `solve’:

public boolean solve(int row, int column) {
    //pass over any numbers that are not empty
    if (getSquare(row, column) != 0) {return moveOn(row, column);}
    else {
        //attempt to find a valid number for the location
        for (int n = 1; n <= L; n++) {
            if (checkRow(row, n) && checkCol(column, n) && checkSquare(row, column, n)) {
                // If a number is allowed at a specific location set that location to the number
                setSquare(row, column, n);
                //Begin checking for a solution based on previous numbers changed           
                if (moveOn(row, column)) {
                    return true;       // solved, yay!
                }
            }               
        }
        //If no number is allowed in this space backtrack to the last successful number 
        //changed and reset all locations that have been changed recursively
        setSquare(row, column, 0);          
    }
    //If the puzzle is unsolveable
    return false;
}