C++ Connect 4 Minimax + Alpha Beta Pruning AI making poor decisions - c++

Question

This post is kind of a follow-up to my last post. To increase the efficiency of a minimax connect-4 AI algorithm, I decided to use alpha-beta pruning. This definitely helped with the long runtime of the program (which I previously believed to be an infinite recursion), but the AI is not working how I want it to.
The AI is simply choosing the next available empty spot to mark, even if it will lead to a loss.
I have tried increasing and decreasing the depth level, and made sure the function that checks for a winner actually works. Furthermore, I converted the 2d vector previously used for the board to a 1d vector, and updated other functions accordingly.
Any help on why the AI is behaving the way it is would be greatly appreciated.
Code:
#include <iostream>
#include <vector>
using namespace std;
bool isFull(std::vector<char>& grid) { //just checks if no empty spaces
for(int i = 0; i < 16; i++) {
if(grid[i] == '-') {
return false;
}
}
return true;
}
pair<bool, char> isWinner(std::vector<char>& grid, char aiMark, char hMark) {
pair<bool, char> temp; // the pair of: whether the game is over, and who won(if any.)
//'X' if AI wins, 'O' if human wins, '-' if tie/game not over.
//horizontal check
for (int i = 0; i < 16; i += 4) {
if (grid[i] == aiMark && grid[i + 1] == aiMark &&
grid[i + 2] == aiMark && grid[i + 3] == aiMark) {
temp.first = true;
temp.second = aiMark;
return temp;
}
else if (grid[i] == hMark && grid[i + 1] == hMark &&
grid[i + 2] == hMark && grid[i + 3] == hMark) {
temp.first = true;
temp.second = hMark;
return temp;
}
}
//vertical check
for (int i = 0; i < 4; i++) {
if (grid[i] == aiMark && grid[i + 4] == aiMark &&
grid[i + 8] == aiMark && grid[i + 12] == aiMark) {
temp.first = true;
temp.second = aiMark;
return temp;
}
else if (grid[i] == hMark && grid[i + 4] == hMark &&
grid[i + 8] == hMark && grid[i + 12] == hMark) {
temp.first = true;
temp.second = hMark;
return temp;
}
}
//diagonal checks
if (grid[0] == aiMark && grid[5] == aiMark &&
grid[10] == aiMark && grid[15] == aiMark) {
temp.first = true;
temp.second = aiMark;
return temp;
}
else if (grid[0] == hMark && grid[5] == hMark &&
grid[10] == hMark && grid[15] == hMark) {
temp.first = true;
temp.second = hMark;
return temp;
}
if (grid[3] == aiMark && grid[6] == aiMark &&
grid[9] == aiMark && grid[12] == aiMark) {
temp.first = true;
temp.second = aiMark;
return temp;
}
else if (grid[3] == hMark && grid[6] == hMark &&
grid[9] == hMark && grid[12] == hMark) {
temp.first = true;
temp.second = hMark;
return temp;
}
if (isFull(grid) == true) {
temp.first = true;
temp.second = '-';
return temp;
}
temp.first = false;
temp.second = '-';
return temp;
}
int minimax(std::vector<char>& grid, int depth, bool maxim,
char aiMark, char hMark, int al, int be) {
pair<bool, char> result = isWinner(grid, aiMark, hMark);
// result.first will be true if game is over, and result.second is:
// 'X' if ai wins, 'O' if human wins, '-' if game is not over or if it ends with tie
if (result.first != false || depth == 0) {
if (result.second == aiMark) {
return depth; // AI wins (maximizing)
}
else if (result.second == hMark) {
return -depth; // Human wins (minimizing)
}
else {
return 0; // Tie or depth = 0
}
}
else {
if (maxim == true) {
int best = INT_MIN;
for (int i = 0; i < 16; i++) {
if (grid[i] == '-') { // is space empty?
grid[i] = aiMark; // editing board
int score = minimax(grid, depth - 1, !maxim, aiMark, hMark, al, be); // call minimax with "new" board
best = max(best, score); // update max
grid[i] = '-'; // backtrack
al = best; // update alpha
if (al >= be) {
break; // pruning
}
}
}
return best; //return max score
}
else {
int worst = INT_MAX;
for (int i = 0; i < 16; i++) {
if (grid[i] == '-') {
grid[i] = hMark;
int score = minimax(grid, depth - 1, !maxim, aiMark, hMark, al, be);
worst = min(worst, score);
grid[i] = '-';
be = worst;
if (be <= al) { //same as the maximizing player but is minimizing instead
break;
}
}
}
return worst; //return min score
}
}
}
void bestMove(std::vector<char>& grid, char aiMark, char hMark) {
int best = INT_MIN; //best score for ai
int finalSpot = -1; //place where ai will put mark
pair<bool, char> result = isWinner(grid, aiMark, hMark); // explained in minimax function comments
if (result.first != false) {
return; // if game is supposed to be over
}
for (int i = 0; i < 16; i++) {
if (grid[i] == '-') {
grid[i] = aiMark;
int score = minimax(grid, 8, true, aiMark, hMark, INT_MIN, INT_MAX);
if (score > best) {
best = score;
finalSpot = i; // update best score and best spot
}
grid[i] = '-'; // backtrack
}
}
grid[finalSpot] = aiMark; // AI finally updates grid
return;
}

Answer 1

The algorithm can choose a losing move, because in bestMove(), you place an aiMark, then call minmax() with maxim set to true which will place a second aiMark in a row. The human does not play after the IA does.
Concerning alpha beta, you can also update alpha with : alpha = max(alpha, best), and the equivalent way with beta. The way you did is not wrong but not optimized, as the value of alpha can drop while it should only raise.
I think the best way to solve your game is to add a transposition table. It is a bit heavy to implement but IA will avoid studying twice the same position. You can first transform your code to a Negamax version which is easy and convenient.