Brute force sudoku solver algorithm in Java problem

Question

Everything seems to work fine in the algorithm besides the solve method. When it executes the program using a solvable Sudoku board, it says that it cannot be solved. I've tried everything I can think of in the solve method. I've tried debugging and it fails after the first row is tested. Any suggestions? Here is the full code so far:

    public class SudokuSolver {

 public static void initializeGrid(int[][] grid, int[][] puzzle) {

  for (int r = 0; r < puzzle.length; r++) {
  for (int c = 0; c < puzzle[0].length; c++) {
  grid [r][c] = puzzle [r][c];
   }
  } 
 }

 public static void displayGrid(int[][] grid) {

  for (int r = 0; r < grid.length; r++) {
   if (r % 3 == 0) {
   System.out.println("+---+---+---+");
   }
   for (int c = 0; c < grid[r].length; c++) {
if (c % 3 == 0) {
 System.out.print("|");
}
displayDigits(r, c, grid);


}
   System.out.print("|");
   System.out.println();
  }
  System.out.println("+---+---+---+");
 }

if (grid[r][c] == 0) {
   System.out.print(' ');
  }
  else {
   System.out.print(grid[r][c]);
  }
 }
 public static int getEmptyCells(int[][] grid, int[][] emptyCells) {
  int i = 0;
  int numEmptyCells = 0;
  for (int r = 0; r < grid.length; r++) {
   for (int c = 0; c < grid[r].length; c++) {
    if (grid[r][c] == 0) {
     emptyCells[i][0] = r;
     emptyCells[i][1] = c;
     numEmptyCells++;
     i++;
    }
   }
  }
  return numEmptyCells;
 }

private static boolean hasNoDuplicates(int[] digitsList) {
  for (int j = 0; j < digitsList.length; j++) {
   for (int k = j + 1; k < digitsList.length; k++) {
    if (digitsList[j] == digitsList[k] && digitsList[j] != 0)
    return false;
   }
  }
  return true;
 }

private static boolean checkCurrentRow(int[][] grid, int currentRow) {

  int[] digitsList = new int[grid.length];
  for (int c = 0; c < digitsList.length; c++) {
   digitsList[c] = grid[currentRow][c];
  }
  if (hasNoDuplicates(digitsList)) {
   return true;
  }
  return false;
 }

private static boolean checkCurrentCol(int[][] grid, int currentCol) {
  int[] digitsList = new int[grid.length];
  for (int i = 0; i < digitsList.length;  i++) {
   digitsList[i] = grid[i][currentCol];
  }
  if (hasNoDuplicates(digitsList)) {
   return true;
  }
  return false;
 }

private static boolean checkCurrentRegion(int[][] grid, int currentRow, int currentCol) {

  int[] digitsList = new int[grid.length];
  currentRow = (currentRow / 3) * 3;
  currentCol = (currentCol / 3) * 3;
  int i = 0;
  for (int r = 0; r < 3; r++) {
   for (int c = 0; c < 3; c++) {
    digitsList[i] = grid[currentRow + r][currentCol + c];
    i++;
   }
  }
  if (hasNoDuplicates(digitsList)) {
   return true;
  }
  return false;
 }

public static boolean isConsistent(int[][] grid, int currentRow, int currentCol) {
  if (checkCurrentRow(grid, currentRow) && checkCurrentCol(grid, currentCol) 
                && checkCurrentRegion(grid, currentRow, currentCol)) {
   return true;
  }
  return false;
 }

public static boolean solvePuzzle(int[][] grid, int[][] emptyCells, int numEmptyCells) {
  int i = 0;
  int j = 0;
  int currentCellDigit = grid[emptyCells[i][0]][emptyCells[i][1]];
  while (j < numEmptyCells) {
   if (currentCellDigit != 9) {
    currentCellDigit++;
    grid[emptyCells[i][0]][emptyCells[i][1]] = currentCellDigit;
    if (isConsistent(grid, emptyCells[i][0], emptyCells[i][1])) {
     grid[emptyCells[i][0]][emptyCells[i][1]] = currentCellDigit;
     i++;
     j++;
    }
    else {
     grid[emptyCells[i][0]][emptyCells[i][1]] = currentCellDigit - 1;
    }
   }
   else {
    currentCellDigit = 0;
    currentCellDigit = grid[emptyCells[i][0]][emptyCells[i][1]];
    i--;
    j--;
    if (j < 0) {
     return false;
    }
   }
  }

return true;

 }

public static void main(String[] args) {

final int SIZE = 9;
  int[][] puzzle  = { {0,2,9,0,0,3,0,0,5},
            {5,0,7,0,0,0,0,9,0},
            {6,0,0,0,0,9,4,2,0},
            {3,0,2,0,0,4,0,0,0},
            {0,0,5,0,3,0,7,0,0},
            {0,0,0,5,0,0,6,0,2},
            {0,9,8,4,0,0,0,0,3},
            {0,3,0,0,0,0,1,0,6},
            {2,0,0,3,0,0,9,4,0}
        };

int[][] grid    = new int[SIZE][SIZE];
  int[][] emptyCellsList = new int[SIZE*SIZE][2];
  int numEmptyCells = 0;

initializeGrid(grid, puzzle);
numEmptyCells = getEmptyCells(grid, emptyCellsList);
 System.out.println("The puzzle:");  
 displayGrid(puzzle);
if (solvePuzzle(grid, emptyCellsList, numEmptyCells)) {
 System.out.println("has been solved:");
   displayGrid(grid);
  }
  else {
   System.out.println("cannot be solved!");
  }
 }

}

Answer 1

Your initializeGrid is wrong. IMHO, it should be:

for (int c = 0; c < puzzle[r].length; c++)

instead of

for (int c = 0; c < puzzle[0].length; c++)

EDIT: my answer below this

It's been indented properly (lesson 1) and arranged the curly braces accordingly (lesson 2). Learn to understand what you are trying to do line by line and when you are stuck on a specific line or method call, look for help (lesson 3). The code below will compile but will not (yet) solve the puzzle. Read and replace the comments inside solvePuzzle for me (lesson 4). Do some thinking and analyzing since this is your homework ;) Good luck!

public class SudokuSolver {
  public static void initializeGrid(int[][] grid, int[][] puzzle) {
   for (int r = 0; r < puzzle.length; r++) {
     for (int c = 0; c < puzzle[0].length; c++) {
       grid [r][c] = puzzle [r][c];
     }
   } 
  }
  
  public static void displayDigits(int r, int c, int[][] grid) {
    if (grid[r][c] == 0) {
      System.out.print('0');
    } else {
      System.out.print(grid[r][c]);
    }
  }

  public static void displayGrid(int[][] grid) {
    for (int r = 0; r < grid.length; r++) {
      if (r % 3 == 0) {
        System.out.println("+---+---+---+");
      }
      for (int c = 0; c < grid[r].length; c++) {
        if (c % 3 == 0) {
          System.out.print("|");
        }
      displayDigits(r, c, grid);
      }
      System.out.print("|");
      System.out.println();
    }
    System.out.println("+---+---+---+");
  }
 
  public static int getEmptyCells(int[][] grid, int[][] emptyCells) {
    int i = 0;
    int numEmptyCells = 0;
    for (int r = 0; r < grid.length; r++) {
       for (int c = 0; c < grid[r].length; c++) {
         if (grid[r][c] == 0) {
           emptyCells[i][0] = r;
           emptyCells[i][1] = c;
           numEmptyCells++;
           i++;
         }
       }
    }
    return numEmptyCells;
  }
  
  private static boolean hasNoDuplicates(int[] digitsList) {
    for (int j = 0; j < digitsList.length; j++) {
      for (int k = j + 1; k < digitsList.length; k++) {
        if (digitsList[j] == digitsList[k] && digitsList[j] != 0)
          return false;
      }
    }
    return true;
  }

  private static boolean checkCurrentRow(int[][] grid, int currentRow) {
    int[] digitsList = new int[grid.length];
    for (int c = 0; c < digitsList.length; c++) {
      digitsList[c] = grid[currentRow][c];
    }
    if (hasNoDuplicates(digitsList)) {
      return true;
    }
    return false;
  }

  private static boolean checkCurrentCol(int[][] grid, int currentCol) {
    int[] digitsList = new int[grid.length];
    for (int i = 0; i < digitsList.length;  i++) {
      digitsList[i] = grid[i][currentCol];
    }
    if (hasNoDuplicates(digitsList)) {
      return true;
    }
    return false;
  }

  private static boolean checkCurrentRegion(int[][] grid, int currentRow,
       int currentCol) {
    int[] digitsList = new int[grid.length];
    currentRow = (currentRow / 3) * 3;
    currentCol = (currentCol / 3) * 3;
    int i = 0;
    for (int r = 0; r < 3; r++) {
      for (int c = 0; c < 3; c++) {
        digitsList[i] = grid[currentRow + r][currentCol + c];
        i++;
      }
    }
    if (hasNoDuplicates(digitsList)) {
      return true;
    }
    return false;
  }

  public static boolean isConsistent(int[][] grid, int currentRow,
       int currentCol) {
    boolean checkRow = checkCurrentRow(grid, currentRow);
    boolean checkCol = checkCurrentCol(grid, currentCol);
    boolean checkReg = checkCurrentRegion(grid, currentRow, currentCol);
    System.out.println("r: " + checkRow + " c: " + checkCol + " r: " + checkReg);
    if (checkRow && checkCol && checkReg) {
      return true;
    }
    return false;
  }

  public static boolean solvePuzzle(int[][] grid, int[][] emptyCells,
      int numEmptyCells) {
    int i = 0;
    int currentCellDigit = 0;
    while (i < numEmptyCells) {
      if (currentCellDigit != 9) {
        // increment cell value
        currentCellDigit++;
        // assign to current cell the current cell value
        //<var> = currentCellDigit;
        System.out.println("Solving---------------------- :" +
            currentCellDigit + " R: " +
            emptyCells[i][0] + " C: " + emptyCells[i][1]);
        // check if value is valid
        if (isConsistent(grid, emptyCells[i][0], emptyCells[i][1])) {
          // reset after setting
          i++;
          currentCellDigit = 0;
        } else {
          // reset cell to zero instead of decrementing it since we're backtracking!
          //grid[emptyCells[i][0]][emptyCells[i][1]] = currentCellDigit - 1;
          //<var> = 0;
        }
      } else {
        // reset current cell to 0
        //<var> = 0;
        // go to previous cell
        i--;
        // exit if theres no cell to backtrack to
        if (i < 0) {
          return false;
        }
        // set previous' cell value as current cell value
        //<var> = grid[emptyCells[i][0]][emptyCells[i][1]];
      }
    }
    return true;
  }
  
  public static void main(String[] args) {
    final int SIZE = 9;
    int[][] puzzle  = { {0,2,9,0,0,3,0,0,5},
                        {5,0,7,0,0,0,0,9,0},
                        {6,0,0,0,0,9,4,2,0},
                        {3,0,2,0,0,4,0,0,0},
                        {0,0,5,0,3,0,7,0,0},
                        {0,0,0,5,0,0,6,0,2},
                        {0,9,8,4,0,0,0,0,3},
                        {0,3,0,0,0,0,1,0,6},
                        {2,0,0,3,0,0,9,4,0}
                    };

    int[][] grid = new int[SIZE][SIZE];
    int[][] emptyCellsList = new int[SIZE*SIZE][2];
    int numEmptyCells = 0;

    initializeGrid(grid, puzzle);
    numEmptyCells = getEmptyCells(grid, emptyCellsList);
    System.out.println("The puzzle:");  
    displayGrid(puzzle);
    if (solvePuzzle(grid, emptyCellsList, numEmptyCells)) {
      System.out.println("has been solved:");
      displayGrid(grid);
    } else {
      System.out.println("cannot be solved!");
    }
  }
}

P.S. this will do backtracking if you've replaced the comments above with proper code.

Answer 2

Think through your code in solvePuzzle. There are several redundant assignments between grid and currentCellDigit. One of i and j is redundant, they have the same value.

Use a debugger to understand what your code is doing.

Better indentation would make your code more readable.

Answer 3

public static boolean solvePuzzle(int[][] grid, int[][] emptyCells,
        int numEmptyCells) {
    int i = 0;
    int currentCellNum = 0;
    while (i < numEmptyCells) {
        currentCellNum = grid[emptyCells[i][0]][emptyCells[i][1]];
        if (currentCellNum != 9) {
            currentCellNum++;
            grid[emptyCells[i][0]][emptyCells[i][1]] = currentCellNum;
            if (isConsistent(grid, emptyCells[i][0], emptyCells[i][1])) {
                i++;
            } 
        } else {
            currentCellNum = 0;
            grid[emptyCells[i][0]][emptyCells[i][1]] = currentCellNum;
            i--;
            if (i < 0) {
                return false;
            }

        }
    }
    return true;
}