If statements causing program to crash with recursion

Question

I am trying to make my program scan the board for move for an AI and the if statement works if I have the first statement by itself but if I add the else if statement and crashes.

void BoardSet::foxNextToSheep(int x, int y, int& moves)
{

    if(board[x+1][y] == 'S' && board[x+2][y] == '.') 
    {
        x = x + 2;
        moves++;
        foxNextToSheep(x, y, moves);
    }
    else if(board[x-1][y] == 'S' && board[x-2][y] == '.')
    {
        x = x - 2;
        moves++;
        foxNextToSheep(x, y, moves);
    }
}

I am assuming that it is due to the recursion, what would be a better way to do this?

I realize it is because board[x-1][y] causes x to become negative and won't work. Is there a way to stop this from happening?

class BoardSet {
public:

//Functions used for foxes
    bool findFox(int x, int y);
    void foxNextToSheep(int x, int y, int& moves);

private: 
    char board[7][8];
    int sheep;
    int foxes;
};

class Foxes {
public:

void foxesProcess(BoardSet& board);
void findFoxes(BoardSet& board);
void checkForPossibleMoves(BoardSet& board);
void print();



private:
    int foxes[2][2];


};


void Foxes::checkForPossibleMoves(BoardSet& board)
{
    int moves1 = 0;
    int foxOneX = foxes[0][0];
    int foxOneY = foxes[0][1];
    board.foxNextToSheep(foxOneX, foxOneY, moves1);
    cout << moves1 << endl;

}

call for check for moves

void Foxes::foxesProcess(BoardSet& board)
{
    cout << "The foxes move: ";
    findFoxes(board);
    checkForPossibleMoves(board);

}

From Main

void processGame(istream& in)
{
    int repeat = 1;

    BoardSet board;
    Sheep sheep;
    Foxes foxes;
    initalizeBoard(in, board);
    while(repeat)
    {
        board.checkForWin(repeat);
        if(repeat == 0)
            break;
        sheep.sheepProcess(board);
        board.print();
        board.checkForWin(repeat);
        if(repeat == 0)
            break;
        foxes.foxesProcess(board);
    }
}

Without more code it is hard to tell but the `x-1` and `x-2` are most likely going out of bounds when `x` is `<=1` — Shafik Yaghmour, Apr 12 '13 at 02:48
What if your forNextToSheep recieves as x argument (or y argument) negative number? — ZarakiKenpachi, Apr 12 '13 at 02:49
If I comment out the else if statement, it works fine. If I add it. the program just says it stopped working unexpectedly. — UnhinderedLimpidity, Apr 12 '13 at 02:49
@ZarakiKenpachi, that is what is happening. Thank you, do you know if there is any way I can keep it from creating negative numbers? — UnhinderedLimpidity, Apr 12 '13 at 02:51
check whether x+1, x+2,x-1,x-2 still inside your array range — Tianyun Ling, Apr 12 '13 at 02:51

Shafik Yaghmour · Accepted Answer · 2013-04-12T07:41:31.060

1

You are not doing any bounds checking in your function so assuming that board is just a standard array or a class that does not bound check then when x<=1 you will go out of bounds and right into undefined behavior.

Without seeing more code the most obvious check and fix would be this:

else if( x > 1 && (board[x-1][y] == 'S' && board[x-2][y] == '.') )

You also have a bounds check issue on the if statement, if x >= 4 then you will be going outside the bounds as well:

if( x <= 4 && (board[x+1][y] == 'S' && board[x+2][y] == '.') )

having some sort of variable that defines max would be helpful in case you decided to change it.

edited Apr 12 '13 at 07:41

answered Apr 12 '13 at 02:54

Shafik Yaghmour

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Better to do boundary check in an upper level since the sequence of if evaluation may not be from left to right. – Tianyun Ling Apr 12 '13 at 02:57
@TianyunLing Unless I am misunderstanding your comment evaluation order is required by the standard, please see this previous thread http://stackoverflow.com/questions/628526/is-short-circuiting-boolean-operators-mandated-in-c-c-and-evaluation-order – Shafik Yaghmour Apr 12 '13 at 03:02
It is still crashing. Should I do the bound check before I do the if statements? – UnhinderedLimpidity Apr 12 '13 at 03:15
@UnhinderedLimpidity It is really hard to say without seeing more code – Shafik Yaghmour Apr 12 '13 at 03:22
@UnhinderedLimpidity Really need to see how it is being called fro main – Shafik Yaghmour Apr 12 '13 at 03:38
@UnhinderedLimpidity updated answer, although without having an example I can run that reproduces the problem it make it hard to fully find all the issues. Which is why you will usually get a better and more accurate answer if you have a piece of code that fully reproduces the problem for us to test. – Shafik Yaghmour Apr 12 '13 at 07:43
@ShafikYaghmour I think I realized what was happening. After it scand through the board going downward, it would then scan going back upward and just keep repeating. Now I just need to figure out how to make it so it doesnt go back up after it scans downward, if you understand what I am saying – UnhinderedLimpidity Apr 12 '13 at 15:16

If statements causing program to crash with recursion

1 Answers1