Improvements?

[ITnerdybird]

This is a code for a tic tac toe game, anyone have any ideas for ways to improve the AI to make the game more challenging for players?

// Tic-Tac-Toe
// Plays the game of tic-tac-toe against a human opponent

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>

using namespace std;

// global constants
const char X = 'X';
const char O = 'O';
const char EMPTY = ' ';
const char TIE = 'T';
const char NO_ONE = 'N';

// function prototypes
void instructions();
char askYesNo(string question);
int askNumber(string question, int high, int low = 0);
char humanPiece();
char opponent(char piece);
void displayBoard(const vector<char>& board);
char winner(const vector<char>& board);
bool isLegal(const vector<char>& board, int move);
int humanMove(const vector<char>& board, char human);
int computerMove(vector<char> board, char computer);
void announceWinner(char winner, char computer, char human);

// main function
int main()
{
    int move;
    const int NUM_SQUARES = 9;
    vector<char> board(NUM_SQUARES, EMPTY);

    instructions();
    char human = humanPiece();
    char computer = opponent(human);
    char turn = X;
    displayBoard(board);

    while (winner(board) == NO_ONE)
    {
        if (turn == human)
        {
            move = humanMove(board, human);
            board[move] = human;
        }
        else
        {
            move = computerMove(board, computer);
            board[move] = computer;
        }
        displayBoard(board);
        turn = opponent(turn);
    }

    announceWinner(winner(board), computer, human);

    return 0;
}

// functions
void instructions()
{
    cout << "Welcome to the ultimate man-machine showdown: Tic-Tac-Toe.\n";
    cout << "--where human brain is pit against silicon processor\n\n";

    cout << "Make your move known by entering a number, 0 - 8.  The number\n";
    cout << "corresponds to the desired board position, as illustrated:\n\n";
    
    cout << "       0 | 1 | 2\n";
    cout << "       ---------\n";
    cout << "       3 | 4 | 5\n";
    cout << "       ---------\n";
    cout << "       6 | 7 | 8\n\n";

    cout << "Prepare yourself, human.  The battle is about to begin.\n\n";
}

char askYesNo(string question)
{
    char response;
    do
    {
        cout << question << " (y/n): ";
        cin >> response;
    } while (response != 'y' && response != 'n');

    return response;
}

int askNumber(string question, int high, int low)
{
    int number;
    do
    {
        cout << question << " (" << low << " - " << high << "): ";
        cin >> number;
    } while (number > high || number < low);

    return number;
}

char humanPiece()
{
    char go_first = askYesNo("Do you require the first move?");
    if (go_first == 'y')
    {
        cout << "\nThen take the first move.  You will need it.\n";
        return X;
    }
    else
    {
        cout << "\nYour bravery will be your undoing... I will go first.\n";
        return O;
    }
}

char opponent(char piece)
{
    if (piece == X)
        return O;
    else
        return X;
}

void displayBoard(const vector<char>& board)
{
    cout << "\n\t" << board[0] << " | " << board[1] << " | " << board[2];
    cout << "\n\t" << "---------";
    cout << "\n\t" << board[3] << " | " << board[4] << " | " << board[5];
    cout << "\n\t" << "---------";
    cout << "\n\t" << board[6] << " | " << board[7] << " | " << board[8];
    cout << "\n\n";
}

char winner(const vector<char>& board)
{
    // all possible winning rows
    const int WINNING_ROWS[8][3] = { {0, 1, 2},
                                     {3, 4, 5},
                                     {6, 7, 8},
                                     {0, 3, 6},
                                     {1, 4, 7},
                                     {2, 5, 8},
                                     {0, 4, 8},
                                     {2, 4, 6} };
    const int TOTAL_ROWS = 8;

    // if any winning row has three values that are the same (and not EMPTY),
    // then we have a winner
    for(int row = 0; row < TOTAL_ROWS; ++row)
    {
        if ( (board[WINNING_ROWS[row][0]] != EMPTY) &&
             (board[WINNING_ROWS[row][0]] == board[WINNING_ROWS[row][1]]) &&
             (board[WINNING_ROWS[row][1]] == board[WINNING_ROWS[row][2]]) )
        {
            return board[WINNING_ROWS[row][0]];
        }
    }

    // since nobody has won, check for a tie (no empty squares left)
    if (count(board.begin(), board.end(), EMPTY) == 0)
        return TIE;

    // since nobody has won and it isn't a tie, the game ain't over
    return NO_ONE;
}

inline bool isLegal(int move, const vector<char>& board)
{
    return (board[move] == EMPTY);
}

int humanMove(const vector<char>& board, char human)
{
    int move = askNumber("Where will you move?", (board.size() - 1));
    while (!isLegal(move, board))
    {
        cout << "\nThat square is already occupied, foolish human.\n";
        move = askNumber("Where will you move?", (board.size() - 1));
    }
    cout << "Fine...\n";
    return move;
}

int computerMove(vector<char> board, char computer)
{
    cout << "I shall take square number ";
    
    // if computer can win on next move, make that move
    for(int move = 0; move < board.size(); ++move)
    {
        if (isLegal(move, board))
        {
            board[move] = computer;
            if (winner(board) == computer)
            {
                cout << move << endl;
                return move;
            }
            // done checking this move, undo it
            board[move] = EMPTY;
        }
    }
        
    // if human can win on next move, block that move
    char human = opponent(computer);
    for(int move = 0; move < board.size(); ++move)
    {
        if (isLegal(move, board))
        {
            board[move] = human;
            if (winner(board) == human)
            {
                cout << move << endl;
                return move;
            }
            // done checking this move, undo it
            board[move] = EMPTY;
        }
    }

    // the best moves to make, in order
    const int BEST_MOVES[] = {4, 0, 2, 6, 8, 1, 3, 5, 7};
    // since no one can win on next move, pick best open square
    for(int i = 0; i < board.size(); ++i)
    {
        int move = BEST_MOVES[i];
        if (isLegal(move, board))
        {
            cout << move << endl;
            return move;
        }
    }
}

void announceWinner(char winner, char computer, char human)
{
	if (winner == computer)
    {
        cout << winner << "'s won!\n";
        cout << "As I predicted, human, I am triumphant once more -- proof\n";
        cout << "that computers are superior to humans in all regards.\n";
		system ("pause");
    }

	else if (winner == human)
    {
        cout << winner << "'s won!\n";
        cout << "No, no!  It cannot be!  Somehow you tricked me, human.\n";
        cout << "But never again!  I, the computer, so swear it!\n";
		system ("pause");
    }

	else
    {
        cout << "It's a tie.\n";
        cout << "You were most lucky, human, and somehow managed to tie me.\n";
        cout << "Celebrate... for this is the best you will ever achieve.\n";
		system ("pause");
	}
}

[ITnerdybird]

bump

[ITnerdybird]

No one knows of any improvements? I am going crazy trying to find any. The AI is already programmed to know the best possible moves. I have no idea how to make it more challenging.

[tntxtnt]

your computerMove() is not efficient. Comp should make decision base on its last move and opponent's last move. Your comp checks all 3x3 squares on the board. What if this is a 50x50 board? Moreover, comp shouldn't be able to modify the board.

also rewrite your winner() function for the same reason: don't list all the winning moves and check all that. Change your params to take board and move (int), and check for that move only.