What's wrong with this Minimax implementation for Tic-Tac-Toe? - c++

Question

I wanted to expand on an older Tic Tac Toe game I made where two players can play versus each other. I want to give the user the option of playing against a difficult AI. The issue is that the AI won't pick the best move all the time. For instance, it will always pick spot 1 if going first. If the user picks spot 2, it will pick spot 4. After this, no matter what the user picks (besides spot 7) the AI won't pick spot 7. Victory for the AI is far from inevitable (the user can still win the game at this point), so that's not the problem.
Any help is appreciated. Thanks!
I'm positive the problem is with my minimax or bestmove functions. It may just be that I haven't properly implemented by minimax function, but I can't spot the issue.
#include <iostream>
#include <iomanip>
#include <string>
#include <array>
// This is a program to play a single game of tic-tac-toe
// between either two human (non-AI) players or an AI.
using namespace std;
void PrintBoard(array <char, 9>);
int programprogress();
int checkwin(array <char, 9>);
int minimax(array <char, 9>, int, int, bool);
int bestMove(array <char, 9>, int);
int Opposite(int);
char PlayerSymbol(int);
const int SIZE = 9;
array <char, SIZE> Pos = { '1', '2', '3', '4', '5', '6', '7', '8', '9' };
int player_number = 1;
int k = -11, result;
bool AI = false, first;
// Global variables used by 2 or more functions.
// Array had to be initialized with numbers instead of blank spaces
// because the check win function wouldn't work properly.
int main()
{
string userinp;
cout << "This is tic tac toe! Here's your board!" << endl;
PrintBoard(Pos);
cout << "Would you like to play versus an AI? (Y/N)" << endl;
cin >> userinp;
if (userinp[0] == 'Y')
{
cout << "Excellent! Would you like to start first, or second? (F/S)" << endl;
cin >> userinp;
if (userinp[0] == 'F')
{
cout << "You will start first!" << endl;
first = false;
player_number = 2;
}
else
{
cout << "The AI will start first!" << endl;
first = true;
}
AI = true;
}
else
{
cout << "Excellent! Your game will start soon." << endl;
}
result = programprogress();
player_number--;
PrintBoard(Pos);
if (result == 1)
cout << endl << "Player " << player_number << " has won!!!\n";
else if (result == 10)
cout << endl << "The AI has won! Better luck next time!\n";
else if (result = -10)
cout << endl << "You beat the world's best AI! Congratulations!\n";
else
cout << endl << "The game has been drawn!" << endl;
return 0;
}
void PrintBoard(array <char, 9> Pos)
{
system("cls");
cout << setw(6) << "|" << setw(6) << "|" << endl << setw(3) << Pos[0] << setw(3) << "|" << setw(3) << Pos[1] << setw(3) << "|" << setw(3) << Pos[2] << " TIC TOE" << endl;
cout << "_____|_____|_____" << endl;
cout << setw(6) << "|" << setw(6) << "|" << endl << setw(3) << Pos[3] << setw(3) << "|" << setw(3) << Pos[4] << setw(3) << "|" << setw(3) << Pos[5] << " TAC " << endl;
cout << "_____|_____|_____" << endl;
cout << setw(6) << "|" << setw(6) << "|" << endl << setw(3) << Pos[6] << setw(3) << "|" << setw(3) << Pos[7] << setw(3) << "|" << setw(3) << Pos[8] << " TIC TOE " << endl;
cout << " | |" << endl;
}
int programprogress()
{
while (k == -11 && AI)
{
bool InvalidChoice = false;
char letter;
//player_number = (player_number % 2) ? 1 : 2;
int PlayerChoice;
if (player_number == 2)
{
cout << endl << "What is your move?" << endl;
cin >> PlayerChoice;
while ((PlayerChoice < 1) || (PlayerChoice > 9))
{
cout << "That's an invalid choice! Please choose a number that is 1-9!" << endl;
cin >> PlayerChoice;
}
PlayerChoice--;
letter = (!first) ? 'X' : 'O';
if (Pos[PlayerChoice] == '1' || Pos[PlayerChoice] == '2' || Pos[PlayerChoice] == '3' || Pos[PlayerChoice] == '4' || Pos[PlayerChoice] == '5' || Pos[PlayerChoice] == '6' || Pos[PlayerChoice] == '7' || Pos[PlayerChoice] == '8' || Pos[PlayerChoice] == '9')
{
Pos[PlayerChoice] = letter;
PrintBoard(Pos);
}
/*else
{
cout << "That space is already taken!" << endl;
player_number--;
}*/
k = checkwin(Pos);
if (k != -11)
k = k * -10;
player_number = 1;
}
else
{
cout << endl << "The computer has made its move!" << endl;
letter = (first) ? 'X' : 'O';
if (first)
PlayerChoice = bestMove(Pos, 1);
else
PlayerChoice = bestMove(Pos, 2);
Pos[PlayerChoice] = letter;
PrintBoard(Pos);
k = checkwin(Pos);
if (k != -11)
k = k * 10;
player_number = 2;
}
}
while (k == -11 && !AI)
{
bool InvalidChoice = false;
char letter;
player_number = (player_number % 2) ? 1 : 2;
int PlayerChoice;
cout << endl << "What's player " << player_number << "'s move?" << endl;
cin >> PlayerChoice;
while ((PlayerChoice < 1) || (PlayerChoice > 9))
{
cout << "That's an invalid choice! Please choose a number that is 1-9!" << endl;
cin >> PlayerChoice;
}
PlayerChoice--;
letter = (player_number == 1) ? 'X' : 'O';
if (Pos[PlayerChoice] == '1' || Pos[PlayerChoice] == '2' || Pos[PlayerChoice] == '3' || Pos[PlayerChoice] == '4' || Pos[PlayerChoice] == '5' || Pos[PlayerChoice] == '6' || Pos[PlayerChoice] == '7' || Pos[PlayerChoice] == '8' || Pos[PlayerChoice] == '9')
{
Pos[PlayerChoice] = letter;
PrintBoard(Pos);
}
else
{
cout << "That space is already taken!" << endl;
player_number--;
}
k = checkwin(Pos);
player_number++;
}
return k;
}
int checkwin(array <char, SIZE> Pos)
{
if (Pos[0] == Pos[1] && Pos[1] == Pos[2])
return 1;
else if (Pos[3] == Pos[4] && Pos[4] == Pos[5])
return 1;
else if (Pos[6] == Pos[7] && Pos[7] == Pos[8])
return 1;
else if (Pos[0] == Pos[3] && Pos[3] == Pos[6])
return 1;
else if (Pos[1] == Pos[4] && Pos[4] == Pos[7])
return 1;
else if (Pos[2] == Pos[5] && Pos[5] == Pos[8])
return 1;
else if (Pos[0] == Pos[4] && Pos[4] == Pos[8])
return 1;
else if (Pos[2] == Pos[4] && Pos[4] == Pos[6])
return 1;
else if (Pos[0] != '1' && Pos[1] != '2' && Pos[2] != '3'
&& Pos[3] != '4' && Pos[4] != '5' && Pos[5] != '6'
&& Pos[6] != '7' && Pos[7] != '8' && Pos[8] != '9')
return 0;
else
return -11;
}
int minimax(array <char, SIZE> newpos, int depth, int player, bool opp)
{
int scale = 0;
if ((player == 1 && first) || (player == 2 && !first))
scale = 10;
else
scale = -10;
//cout << scale;
int score = scale*checkwin(newpos);
if (score < 0)
score += depth;
else if (score > 0)
score -= depth;
if (score == -10 || score == 10 || score == 0)
return score;
if (opp)
{
int best = -1000;
for (int i = 0; i < SIZE; i++)
{
if (newpos[i] != 'X' && newpos[i] != 'O')
{
char temp = newpos[i];
newpos[i] = PlayerSymbol(player);
best = max(best, minimax(newpos, depth + 1, Opposite(player), !opp));
newpos[i] = temp;
}
}
return best;
}
else
{
int best = 1000;
for (int i = 0; i < SIZE; i++)
{
if (newpos[i] != 'X' && newpos[i] != 'O')
{
char temp = newpos[i];
newpos[i] = PlayerSymbol(player);
best = min(best, minimax(newpos, depth + 1, Opposite(player), !opp));
newpos[i] = temp;
}
}
return best;
}
}
int bestMove(array <char, SIZE> newpos, int player)
{
int best = -1000;
int bestpos = -1;
for (int i = 0; i < SIZE; i++)
{
if (newpos[i] != 'X' && newpos[i] != 'O')
{
char temp = newpos[i];
newpos[i] = PlayerSymbol(player);
int move = minimax(newpos, 0, player, !first);
newpos[i] = temp;
if (move > best)
{
//cout << "I like pineapple on pizza" << endl;
bestpos = i;
best = move;
}
/*if (move == best)
{
cout << "I like pineapple on pizza" << endl;
}*/
}
}
cout << bestpos;
return bestpos;
}
int Opposite(int x)
{
if (x == 1)
return 2;
else
return 1;
}
char PlayerSymbol(int x)
{
if (x == 1)
return 'X';
else
return 'O';
}
An out of bounds error due to the -1 value of bestpos. I'm not sure how to change this, though.

Answer 1

There are 4 issues that I could find. Solving them seems to lead to the intended behavior.
Firstly, when you call minimax(newpos, 0, player, !first); from inside the bestMove function, you pass player rather than Opposite(player), indicating that the first minimax step will be performed by the same player as the bestMove step. In other words: The AI makes two successive moves for itself. Therefore player needs to be changed to Opposite(player).
Secondly, minimax has a bool variable named opp that seems to indicate whether it is the AI or its opponent making the move. For the first minimax step, opp is set to !first, indicating that only if the AI goes first, the opponent will make a move after the AI. That is incorrect. It is always the opponent making a move after the AI. So bestMove should call minimax with true rather than !first. As an aside, opp is redundant, because you can use (player == 1 && first) || (player == 2 && !first) to check whether it's the AI or its opponent making the move.
Thirdly, the scale is set the wrong way around. With (player == 1 && first) || (player == 2 && !first) you check whether its the AI making a move. But you do that in the next minimax step, after the potentially winning move. So if the AI is making a move and the game is already won, then the opponent made the winning move, not the AI. Ergo, the scale should be
if ((player == 1 && first) || (player == 2 && !first))
scale = -10;
else
scale = 10;
instead.
Lastly, you check whether the score is 10 or -10 after adding the depth. If depth is not 0, then this check will always fail. So beyond depth 0, the AI can only see a draw, never a win. You could instead write
if (score == -10 || score == 10 || score == 0)
{
if (score < 0)
score += depth;
else if (score > 0)
score -= depth;
return score;
}
Hope this answers your question fully.