Return bestMove in minimax algorithm for tictactoe

Question

I have tried to code the minimax algorithm for tic-tac-toe given in Russel Norvig\'s book on Artificial Intelligence. It had everything except that the way to return the bes

Answer 1

Well, it looks like MiniMax correctly chooses it for you, just call it with an initial state and a depth. (Unless the first player according to the state is the second player, then you should call min_move in MiniMax.)

EDIT: yes, I overlooked something, bestMove currently does not make much sense. In the program within max_move you change the loop like this:

for(int i = 0 ; i < nMoves ; i++)
{
    moveT move = moveList[i];
    MakeMove(state, move);

    int new_value = min_move(state, depth+1);
    if(new_value > v)
    {
      v=new_value;
    }
    RetractMove(state, move);

}

After that you can think about you what bestMove means? My idea is that you are interested in finding one of the "best possible" series of moves for tic-tac-toe. For that you need a vector or even better a stack. But that also means having std::stack* best_moves as the last parameter.

For the stack implementation, in min_move you return the next moves and if their value is the best, you will push your move on the top of the best_moves stack. Of course at the end of the game you just return the empty stack. It takes an OOP approach to pull it off properly, I'll do it when I have some time.

If all you need is merely the best next move then I suggest you change the return types of min_move and max_moe to some struct like this:

struct Value_move{
  int value;
  moveT best_move;
};

Then the new implementation of max_move looks like the following:

const int MOVE_INVALID = -12345;
const int MOVE_NOTHING = -12346;

Value_move max_move(stateT state, int depth)
{
    Value_move best;
    best.value = -10000; best.best_move = MOVE_INVALID;

    if(GameIsOver(state))
    {
        best.value = EvaluateStaticPosition(state);
        best.best_move = MOVE_NOTHING;
        return best;
    }

    vector moveList;
    GenerateMoveList(state, moveList);
    int nMoves = moveList.size();

    for(int i = 0 ; i < nMoves ; i++)
    {
        moveT move = moveList[i];
        MakeMove(state, move);
        Value_move curr = min_move(state, depth+1);
        if(curr.value > best.value)
        {
            best.value = curr.value;
            best.best_move = move;
        }
        RetractMove(state, move);

    }

    return v;

}

You only need to pick up the best_move field in the returned struct in the MiniMax function.

REMARK:
You have to admit though this does not resemble a c++ program in many aspects but rather a c program. Otherwise, all the functions in CapitalCamelCase should be class methods, you should pass states by (const) ref instead of value -- but this whole code makes sense only if the status is really a pointer behind a typedef.