Thread: Sudoku Solution Generator

I know what is going wrong with the program but I am honestly lost on how to fix it. The problem is that is eventually getting stuck because there is no possible place to put a number. I would really appreciate some help on how to fix this problem, or even just some hints to guide me in the right direction please.
The main file.

package sudoku;
public class Sudoku {
 
    public static void main(String[] args) {
        //creating the arrays that will store the solution and the puzzle
        int solution[][] = new int[9][9];
        int puzzle[][] = new int[9][9];
        //creating the object
        SudokuMaker newSudoku = new SudokuMaker();
        //this asks if you want directions
        newSudoku.Directions();
        //this asks the difficulty of puzzle you want
        newSudoku.Difficulty();
        //this sends solution to a method to create the solution
        newSudoku.Solution(solution);
        //this sets puzzle and solution equal to each other
        //switch tester with solution and comment 
        //out newSudoku.Soltuion and newSudoku.PrintArray(solution)
        //since the Soltuion method doesn't always work
        newSudoku.Equalizer(solution,puzzle);
        //this takes out numbers making it a puzzle
        newSudoku.PuzzleMaker(puzzle);
        //This prints them out
        System.out.println("Here is the puzzle.");
        newSudoku.PrintArray(puzzle);
        System.out.println("Here is the solution.");
        newSudoku.PrintArray(solution);
    }
}

The class file.

package sudoku;
import java.util.*;
 
public class SudokuMaker {
    //This is so I can use random numbers later on
    Random randomnum = new Random();
    //So I can ask questions of the user
    Scanner reader = new Scanner(System.in);
    //my state variables
    int x;
 
    //this method asks if you want directions or not by using Scanner with a string
    //if you say yes it prints the directions if no it does nothing
    //if you do something else it ends the program
    public void Directions(){
        String choice;
        System.out.println("Do you want the directions? Yes or no.");
        choice=reader.nextLine();
        if (choice.equalsIgnoreCase("Yes") || choice.equalsIgnoreCase("Y")){
            System.out.println("This program will print out a Sudoku puzzle and answer for you.\nIn the puzzle it will will have numbers placed in a 9 by 9 box.\nThere will be columns, rows and 9 3 by 3 boxes.\nYou can only have one copy of the numbers 1 through 9 in each column, row, and 3 by 3 box.\nThere will be boxes with the number 0 in them.\nThis is where you put a number that could possibly be there.\nThis program will aslo tell you the solution.");
        }
        else if(choice.equalsIgnoreCase("No")||choice.equalsIgnoreCase("N")){
 
        }
        else{
            System.out.println("That was not an option. No Sudoku for you. Now ending program.");
            System.exit(0);
        }
    }
 
    //this program asks what difficulty you want
    //choice 1 is easy and choice 2 is hard
    //this uses a scanner and int to read that
    //there is a while loop in there for if in case you pick something other than 1 or 2 it ends the program
    public void Difficulty(){
        boolean validInput=false;
        System.out.println("Pick a difficulty.\n1. Easy\n2. Hard");
        while(!validInput){
            try{
                x=reader.nextInt();
                validInput=true;
                if(x>3){
                    System.out.println("That was not an option. No Sudoku for you. Now ending program.");
                    System.exit(0);
                }
                else if(x<1){
                    System.out.println("That was not an option. No Sudoku for you. Now ending program.");
                    System.exit(0);
                }
            }
            catch(InputMismatchException e){
                System.out.println("That was not an option. No Sudoku for you. Now ending program.");
                System.exit(0);
            }
        }
    }
 
    //this program sets up a nested loop that will print any 9by9 array sent to it
    public void PrintArray(int k[][]) {
        for (int row = 0; row < 9; row++) {
            for (int col = 0; col < 9; col++) {
                System.out.print(k[row][col] + "\t");
            }
            System.out.println(" ");
        }
    }
 
    //this program makes the solution
    //it does this by having two integers one for columns and one for rows
    //it then calls a method which places all the numbers
    //it does this by using a loop from 1-9 and sending it the number that the loop is using
    //the it returns the solution
    public int[][] Solution(int sol[][]) {
        int col=0;
        int row=0;
        for(int num=1;num<10;num++){
            NumberPlacer(num,sol);
        }
        return sol;
    }
 
    //this says if there is a number greater than zero do not put a number here
    //this makes it so if a number was already placed somewhere it won't be overwritten
    public boolean Forbidden(int row, int col,int sol[][]){
        if(sol[row][col]>0){
            return true;
        }
        return false;
    }
 
    //this method does most of the work
    //it starts with box 5(the middle box) and puts a number there
    //it then goes to the boxes on the left(box 4)and puts a number there while also making sure it is possible to put the number there
    //after that it does the same for the box to the right(box 6), above(box 2), and below(box 7) of box 5
    //it then does the four corner boxes, box 1, box 3, box 7, box 9
    //it does this for all numbers sent which is one through 9
    public void NumberPlacer(int num, int sol[][]){
        int col=0;
        int row=0;
        //box5
        while(true){
            row=RowSet2(row);
            col=ColSet2(col);
            while(Forbidden(row,col,sol)){
                row=RowSet2(row);
                col=ColSet2(col);
            }
            sol[row][col]=num;
            if(!CheckCol(col,row,num,sol)||!CheckRow(row,col,num,sol)){
                sol[row][col]=0;
            } 
            else if(CheckCol(col,row,num,sol)&&CheckRow(row,col,num,sol)){
                break;
            }
        }
        //box4
        while(true){
            row=RowSet2(row);
            col=ColSet1(col);
            while(Forbidden(row,col,sol)){
                row=RowSet2(row);
                col=ColSet1(col);
            }
            sol[row][col]=num;
            if(!CheckRow(row,col,num,sol)){
                sol[row][col]=0;
            } 
            else if(CheckRow(row,col,num,sol)){
                break;
            }
        }
        //box6
        while(true){
            row=RowSet2(row);
            col=ColSet3(col);
            while(Forbidden(row,col,sol)){
                row=RowSet2(row);
                col=ColSet3(col);
            }
            sol[row][col]=num;
            if(!CheckRow(row,col,num,sol)){
                sol[row][col]=0;
            } 
            else if(CheckRow(row,col,num,sol)){
                break;
            }
        }
        //box2
        while(true){
            row=RowSet1(row);
            col=ColSet2(col);
            while(Forbidden(row,col,sol)){
                row=RowSet1(row);
                col=ColSet2(col);
            }
            sol[row][col]=num;
            if(!CheckCol(col,row,num,sol)){
                sol[row][col]=0;
            } 
            else if(CheckCol(col,row,num,sol)){
                break;
            }
        }
        //box8
        while(true){
            row=RowSet3(row);
            col=ColSet2(col);
            while(Forbidden(row,col,sol)){
                row=RowSet3(row);
                col=ColSet2(col);
            }
            sol[row][col]=num;
            if(!CheckCol(col,row,num,sol)){
                sol[row][col]=0;
            } 
            else if(CheckCol(col,row,num,sol)){
                break;
            }
        }
        //box1
        while(true){
            row=RowSet1(row);
            col=ColSet1(col);
            while(Forbidden(row,col,sol)){
                row=RowSet1(row);
                col=ColSet1(col);
            }
            sol[row][col]=num;
            if(!CheckCol(col,row,num,sol)||!CheckRow(row,col,num,sol)){
                sol[row][col]=0;
            } 
            else if(CheckCol(col,row,num,sol)&&CheckRow(row,col,num,sol)){
                break;
            }
        }
        //box3
        while(true){
            row=RowSet1(row);
            col=ColSet3(col);
            while(Forbidden(row,col,sol)){
                row=RowSet1(row);
                col=ColSet3(col);
            }
            sol[row][col]=num;
            if(!CheckCol(col,row,num,sol)||!CheckRow(row,col,num,sol)){
                sol[row][col]=0;
            } 
            else if(CheckCol(col,row,num,sol)&&CheckRow(row,col,num,sol)){
                break;
            }
        }
        //box7
        while(true){
            row=RowSet3(row);
            col=ColSet1(col);
            while(Forbidden(row,col,sol)){
                row=RowSet3(row);
                col=ColSet1(col);
            }
            sol[row][col]=num;
            if(!CheckCol(col,row,num,sol)||!CheckRow(row,col,num,sol)){
                sol[row][col]=0;
            } 
            else if(CheckCol(col,row,num,sol)&&CheckRow(row,col,num,sol)){
                break;
            }
        }
        //box9
        while(true){
            row=RowSet3(row);
            col=ColSet3(col);
            while(Forbidden(row,col,sol)){
                row=RowSet3(row);
                col=ColSet3(col);
            }
            sol[row][col]=num;
            if(!CheckCol(col,row,num,sol)||!CheckRow(row,col,num,sol)){
                sol[row][col]=0;
            } 
            else if(CheckCol(col,row,num,sol)&&CheckRow(row,col,num,sol)){
                break;
            }
        }
    }
 
    //this is for the first row of boxes or rows 0-2
    public int RowSet1(int row){
        row=randomnum.nextInt(3);
        return row;
    }
 
    //this is for the second row of boxes or rows 3-5
    public int RowSet2(int row){
        row=randomnum.nextInt(3)+3;
        return row;
    }
 
    //this is for the third row of boxes or rows 6-8
    public int RowSet3(int row){
        row=randomnum.nextInt(3)+6;
        return row;
    }
 
    //this is for the first cols of boxes or cols 0-2
    public int ColSet1(int col){
        col=randomnum.nextInt(3);
        return col;
    }
 
    //this is for the second cols of boxes or cols 3-5
    public int ColSet2(int col){
        col=randomnum.nextInt(3)+3;
        return col;
    }
 
    //this is for the first cols of boxes or cols 6-8
    public int ColSet3(int col){
        col=randomnum.nextInt(3)+6;
        return col;
    }
 
    //this checks the row for the number it is sent and makes sure it is not in that row other than that spot the number is in
    public boolean CheckRow(int row, int y, int num, int sol[][]) {
        for (int col = 0; col < 9; col++) {
            if((row==row)&&(col==y)){
                continue;
            }
            if (sol[row][col] == num) {
                return false;
            }
        }
        return true;
    }
 
    //this checks the col for the number it is sent and makes sure it is not in that col other than that spot the number is in
    public boolean CheckCol(int col,int x, int num, int sol[][]) {
        for (int row = 0; row < 9; row++) {
            if((col==col)&&(row==x)){
                continue;
            }
            if (sol[row][col] == num) {
                return false;
            }
        }
        return true;
    }
 
    //this is to set two 9by9 arrays equal to each other
    public int[][] Equalizer(int sol[][], int puzz[][]){
        for(int row=0;row<9;row++){
            for(int col=0;col<9;col++){
                puzz[row][col]=sol[row][col];
            }
        }
        return puzz;
    }
 
    //this creates the puzzle depending on what difficulty you chose
    //it chooses random spots to set too 0
    //these are still solvable 
    public int[][] PuzzleMaker(int puzz[][]){
        int numberRemoved=0;
        int row=0;
        int col=0;
        if(x==1){
            //box1
            while(numberRemoved<4){
                row=RowSet1(row);
                col=ColSet1(col);
                while(puzz[row][col]==0){
                    row=RowSet1(row);
                    col=ColSet1(col);
                }
                puzz[row][col]=0;
                numberRemoved++;
            }
            numberRemoved=0;
            //box2
            while(numberRemoved<4){
                row=RowSet1(row);
                col=ColSet2(col);
                while(puzz[row][col]==0){
                    row=RowSet1(row);
                    col=ColSet2(col);
                }
                puzz[row][col]=0;
                numberRemoved++;
            }
            numberRemoved=0;
            //box3
            while(numberRemoved<4){
                row=RowSet1(row);
                col=ColSet3(col);
                while(puzz[row][col]==0){
                    row=RowSet1(row);
                    col=ColSet3(col);
                }
                puzz[row][col]=0;
                numberRemoved++;
            }
            numberRemoved=0;
            //box4
            while(numberRemoved<4){
                row=RowSet2(row);
                col=ColSet1(col);
                while(puzz[row][col]==0){
                    row=RowSet2(row);
                    col=ColSet1(col);
                }
                puzz[row][col]=0;
                numberRemoved++;
            }
            numberRemoved=0;
            //box5
            while(numberRemoved<4){
                row=RowSet2(row);
                col=ColSet2(col);
                while(puzz[row][col]==0){
                    row=RowSet2(row);
                    col=ColSet2(col);
                }
                puzz[row][col]=0;
                numberRemoved++;
            }
            numberRemoved=0;
            //box6
            while(numberRemoved<4){
                row=RowSet2(row);
                col=ColSet3(col);
                while(puzz[row][col]==0){
                    row=RowSet2(row);
                    col=ColSet3(col);
                }
                puzz[row][col]=0;
                numberRemoved++;
            }
            numberRemoved=0;
            //box7
            while(numberRemoved<4){
                row=RowSet3(row);
                col=ColSet1(col);
                while(puzz[row][col]==0){
                    row=RowSet3(row);
                    col=ColSet1(col);
                }
                puzz[row][col]=0;
                numberRemoved++;
            }
            numberRemoved=0;
            //box8
            while(numberRemoved<4){
                row=RowSet3(row);
                col=ColSet2(col);
                while(puzz[row][col]==0){
                    row=RowSet3(row);
                    col=ColSet2(col);
                }
                puzz[row][col]=0;
                numberRemoved++;
            }
            numberRemoved=0;
            //box9
            while(numberRemoved<4){
                row=RowSet3(row);
                col=ColSet3(col);
                while(puzz[row][col]==0){
                    row=RowSet3(row);
                    col=ColSet3(col);
                }
                puzz[row][col]=0;
                numberRemoved++;
            }
        }
        else if(x==2){
           //box1
            while(numberRemoved<5){
                row=RowSet1(row);
                col=ColSet1(col);
                while(puzz[row][col]==0){
                    row=RowSet1(row);
                    col=ColSet1(col);
                }
                puzz[row][col]=0;
                numberRemoved++;
            }
            numberRemoved=0;
            //box2
            while(numberRemoved<5){
                row=RowSet1(row);
                col=ColSet2(col);
                while(puzz[row][col]==0){
                    row=RowSet1(row);
                    col=ColSet2(col);
                }
                puzz[row][col]=0;
                numberRemoved++;
            }
            numberRemoved=0;
            //box3
            while(numberRemoved<5){
                row=RowSet1(row);
                col=ColSet3(col);
                while(puzz[row][col]==0){
                    row=RowSet1(row);
                    col=ColSet3(col);
                }
                puzz[row][col]=0;
                numberRemoved++;
            }
            numberRemoved=0;
            //box4
            while(numberRemoved<5){
                row=RowSet2(row);
                col=ColSet1(col);
                while(puzz[row][col]==0){
                    row=RowSet2(row);
                    col=ColSet1(col);
                }
                puzz[row][col]=0;
                numberRemoved++;
            }
            numberRemoved=0;
            //box5
            while(numberRemoved<5){
                row=RowSet2(row);
                col=ColSet2(col);
                while(puzz[row][col]==0){
                    row=RowSet2(row);
                    col=ColSet2(col);
                }
                puzz[row][col]=0;
                numberRemoved++;
            }
            numberRemoved=0;
            //box6
            while(numberRemoved<5){
                row=RowSet2(row);
                col=ColSet3(col);
                while(puzz[row][col]==0){
                    row=RowSet2(row);
                    col=ColSet3(col);
                }
                puzz[row][col]=0;
                numberRemoved++;
            }
            numberRemoved=0;
            //box7
            while(numberRemoved<5){
                row=RowSet3(row);
                col=ColSet1(col);
                while(puzz[row][col]==0){
                    row=RowSet3(row);
                    col=ColSet1(col);
                }
                puzz[row][col]=0;
                numberRemoved++;
            }
            numberRemoved=0;
            //box8
            while(numberRemoved<5){
                row=RowSet3(row);
                col=ColSet2(col);
                while(puzz[row][col]==0){
                    row=RowSet3(row);
                    col=ColSet2(col);
                }
                puzz[row][col]=0;
                numberRemoved++;
            }
            numberRemoved=0;
            //box9
            while(numberRemoved<5){
                row=RowSet3(row);
                col=ColSet3(col);
                while(puzz[row][col]==0){
                    row=RowSet3(row);
                    col=ColSet3(col);
                }
                puzz[row][col]=0;
                numberRemoved++;
            }
        }
 
        return puzz;
    }
}

Thanks everyone for your help.

Welcome to the forum! Thank you for taking the time to learn to post code correctly. If you haven't already, please read this topic to learn other useful info for new members.

Where does it appear to be getting stuck? If you're not sure, add prints throughout the program to determine how far it gets and where it is getting stuck. Once you know that, find the specific code, whether it's a loop or recursive logic - whatever it is, that is causing the program to become stuck. Then work that code to determine why it is not escaping. Add additional prints to get the variable values at the point it gets stuck and determine why those values are preventing progress.

Your program is commented at the method level (thanks), but I don't see an overall description of what it does. It looks like:

It gets the difficulty level from the user
Generates a puzzle

And after that I get lost. I don't understand the output of the puzzle generator. Is an entire puzzle created, all filled in, and stored as the solution? Is then that solution presented to the user with some of the numbers removed (set to zero) as the puzzle to solve?

The NumberPlacer() method is also confusing. Can you describe how that works? How is it determined that a valid and solvable puzzle has been created?

I did have print statements in there. Basically it asks if you want directions or not, it then asks the difficulty which you understood. I then have it set up so a method has a different method in a loop such that starting with number 1 it will put a 1 in each three by three box. Then it does the 2;s and 3's and so on up till and including the number 9. I did have print statements earlier on and where it get's is completely random every time. As it goes through putting the numbers there it checks the row and column to see if this number is allowed there. If it isn't it puts it in a different spot, however sometimes there are no other spots it can put the number. The program then takes the solution(provided it finished) and depending on the difficulty takes some numbers out to create the puzzle. The puzzle method works I have checked with several different hard coded Sudoku solutions. The problem is that a solution is not always being made and it gets stuck on a number and the code never finishes.

Okay thank you very much. I will try to come up with a way to shuffle the rows and columns like the way they show in the link you put.