Tic tac toe Minimax Algorithm Having Weird Behavior (C++) - c++

Question

The other day, I wrote a console game of Tic-Tac-Toe in c++ for my son. He wanted me to add a computer, and I ended us using the minimax algorithm for the first time. I did some quick testing, but really just gave my laptop to my son as soon as it was printing stuff, who played with it for a couple minuets. I looked over his sholder once or twice, and noticed that it wasn't playing optimally, iv'e been trying to debug it, but I can't see where it goes wrong. I tried getting rid of alpha beta prunning, but that did not change anything.
For context, on the board the computer is -1, blank is 0, and the player is 1.
Here is the minimax function:
int minimax(int board[9], int depth, int alpha, int beta, bool isMaxizimaizingPlayer)
{
bool found = false;
for (int i = 0; i < 9; i++)
{
if (board[i] == 0)
{
found = true;
}
}
if (!found)
{
return eval(board);
}
if (depth == 0 || eval(board) != 0)
{
return eval(board);
}
if (isMaxizimaizingPlayer)
{
int maxEval = -2;
for (int spot = 0; spot < 9; spot++)
{
if (board[spot] == 0)
{
board[spot] = 1;
int e = minimax(board, depth - 1, alpha, beta, false);
if (e > maxEval)
{
maxEval = e;
}
//if (beta < alpha)
//{
// break;
//}
board[spot] = 0;
}
}
return maxEval;
}
else {
int minEval = 2;
for (int spot = 0; spot < 9; spot++)
{
if (board[spot] == 0)
{
board[spot] = -1;
int e = minimax(board, depth - 1, alpha, beta, true);
if (e < minEval)
{
minEval = e;
}
//if (beta < alpha)
//{
// break;
//}
board[spot] = 0;
}
}
return minEval;
}
}
To be compleate, here is my eval function:
int eval(int board[9])
{
/*horizontial*/
for (int i = 0; i < 3; i++)
{
if (board[i * 3] == board[i * 3 + 1] && board[i * 3 + 2] == board[i * 3] && board[i * 3] != 0)
{
return board[i * 3];
}
}
/*vertical*/
for (int i = 0; i < 3; i++)
{
if (board[i] == board[i + 3] && board[i] == board[i + 6] && board[i] != 0)
{
return board[i];
}
}
/*Both diags*/
if (board[4] != 0) {
if (board[0] == board[4] && board[0] == board[8])
{
return board[4];
}
if (board[2] == board[4] && board[4] == board[6])
{
return board[4];
}
}
return 0;
}
And here is the inital call:
int spot = 0;
int minEval = 2;
for (int i = 0; i < 9; i++)
{
if (board[i] == 0)
{
board[i] = -1;
int score = minimax(board, 3, -2, 2, false);
if (score < minEval) {
minEval = score;
spot = i;
}
board[i] = 0;
}
}
std::cout << "The computer went in spot " << spot + 1 << std::endl;
board[spot] = -1;
printBoard(board);

Answer 1

It looks like you only call minimax with a depth of three, so the algorithm will only look up to three moves ahead, if you want optimal play you need to set the depth to > 9, so that the agent is always looking ahead to the end of the game.