Question

thats my first question here and i hope someone can help me.
I´m new at the iPhone programming and want to try an easy app…
It´s an SudokuSolver which is working with an recursive Method. In JAVA this code is making no problems, but in Objective-C the code isn´t stopping when Sudoku is solved. It´s still trying to solve the Sudoku and stops later.

Anyone an idea?!

Here´s the code.

- (SudokuSolver *) initWithField: (int[9][9]) field {
self = [super init];
if(self) {
    for (int i=0; i<9; i++) {
        for (int j=0; j<9; j++) {
            sudokuField[i][j] = field[i][j];
            if (field[i][j]) {
                sudokuFieldStatic[i][j] = 1;
            } else {
                sudokuFieldStatic[i][j] = 0;
            }
        }
    }
}
return self;
}

- (void) solve {
   [self solveFieldAtRow:0 andCol:0];
}

- (void) solveFieldAtRow: (int) row andCol: (int) col {
  if (row > 8) {
    return;
  } else {
    while (sudokuField[row][col] != 0) {
        if (++col > 8) {
            col = 0;
            row++;
            if (row > 8) {
                return;
            }
        }
    }
    for (int num=1; num<10; num++) {
        if ([self checkRow:row forNumber:num] && [self checkCol:col forNumber:num] && [self checkFieldAtRow:row andCol:col forNumber:num]) {
            sudokuField[row][col] = num;
            [self showFieldInConsole:0];
            if (col < 8) {
                [self solveFieldAtRow:row andCol:col+1];
            } else {
                [self solveFieldAtRow:row+1 andCol:0];
            }
        }
    }
    sudokuField[row][col] = 0;
  }
}

Answer 1

The code isn’t stopping when the puzzle is solved because you don’t check whether the puzzle is solved after the recursive call. So even if the recursive call found a solution, the code just continues on even after finding a solution until it has tried every possibility.

Since you say you have Java code that works, I suggest you compare the logic of the Java program versus this code. You’ll probably find the Java code does include such a test.

---

Edit From your comment above, I see that you won’t find such a test in your Java code, because there you are abusing exceptions to “return” from the recursion when a solution is found. The proper way is to have each recursive call return a true value if it found a solution and false if it didn’t. And then each step should check if its child call succeeded, and itself return success if so. Something like this:

- (BOOL) solveFieldAtRow: (int) row andCol: (int) col {
    if (row > 8) {
        // reached the end, so it must have succeeded
        return YES;
    } else {
        while (sudokuField[row][col] != 0) {
            if (++col > 8) {
                col = 0;
                row++;
                if (row > 8) {
                    // reached the end, so it must have succeeded
                    return YES;
                }
            }
        }
        for (int num=1; num<10; num++) {
            if ([self checkRow:row forNumber:num] && [self checkCol:col forNumber:num] && [self checkFieldAtRow:row andCol:col forNumber:num]) {
                sudokuField[row][col] = num;
                [self showFieldInConsole:0];
                BOOL result;
                if (col < 8) {
                    result = [self solveFieldAtRow:row andCol:col+1];
                } else {
                    result = [self solveFieldAtRow:row+1 andCol:0];
                }
                if (result) {
                    // Our child call succeeded, so we pass that back up
                    // the stack.
                    return YES;
                }
            }
        }
        sudokuField[row][col] = 0;
        // If we get here, we could not find a solution. Return failure
        // back up the stack.
        return NO;
    }
}