Problem with recursive objective-c void-method - objective-c

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;
}
}