TicTacToe.cpp

#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 definitions
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>* const pBoard);
char winner(const vector<char>* const pBoard);
bool isLegal(const vector<char>* const pBoard, int move);
int humanMove(const vector<char>* const pBoard, char human);
int computerMove(vector<char> board, char computer);
void announceWinner(char winner, char computer, char human);
  
  
  
  
int main()
{
      
    int move;
    char play_again = ' ';
    do{
    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);
    play_again = askYesNo("Do you want to play again?");
    }while(play_again == 'y');
    cout << "Thank you for playing.";
    return 0;
}
  
//functions
void instructions()
{
    cout << "Welcome to TicTacToe.\n";
    cout << "make your move known by enter 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";
  
}
  
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 want to go first?");
    if( go_first == 'y')
    {
        cout << "\nMake your first move.";
        return X;
    }
    else
    {
        cout << "\nThe computer will make the first move.";
        return O;
    }
}
  
char opponent(char piece)
{
    if (piece == X)
        return O;
    else
        return X;
}
  
void displayBoard(const vector<char>* const pBoard)
{
    cout << "\n\t" << (*pBoard)[0] << " | " << (*pBoard)[1] << " | " << (*pBoard)[2];
    cout << "\n\t" << "---------";
    cout << "\n\t" << (*pBoard)[3] << " | " << (*pBoard)[4] << " | " << (*pBoard)[5];
    cout << "\n\t" << "---------";
    cout << "\n\t" << (*pBoard)[6] << " | " << (*pBoard)[7] << " | " << (*pBoard)[8];
    cout << "\n\t";
}
  
char winner(const vector<char>* const pBoard)
{
    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;
  
  
    for (int row = 0; row < TOTAL_ROWS; ++row)
    {
        if( ((*pBoard)[WINNING_ROWS[row][0]] != EMPTY) &&
            ((*pBoard)[WINNING_ROWS[row][0]] == (*pBoard)[WINNING_ROWS[row][1]]) &&
            ((*pBoard)[WINNING_ROWS[row][1]] == (*pBoard)[WINNING_ROWS[row][2]]) )
        {
            return (*pBoard)[WINNING_ROWS[row][0]];
        }
    }
  
    if (count(pBoard->begin(), pBoard->end(), EMPTY) == 0)
        return TIE;
  
    return NO_ONE;
}
  
inline bool isLegal(int move, const vector<char>* pBoard)
{
    return ((*pBoard)[move] == EMPTY);
}
  
int humanMove(const vector<char>* const pBoard, char human)
{
    int move = askNumber("Where will you move?", (pBoard->size() - 1));
    while(!isLegal(move, pBoard))
    {
        cout << "\nThat is not a valid move.\n";
        move = askNumber("Where will you move?", (pBoard->size() - 1));
    }
    cout << "Fine...\n";
    return move;
}
  
int computerMove(vector<char> board, char computer)
{
    cout << "I shall take square number ";
  
    for(int move = 0; move < board.size(); ++move)
    {
        if(isLegal(move, &board))
        {
            board[move] = computer;
            if (winner(&board) == computer)
            {
                cout << move << endl;
                return move;
            }
  
            board[move] = EMPTY;
        }
    }
  
    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;
            }
  
            board[move] = EMPTY;
        }
    }
  
    const int BEST_MOVES[] = {4, 0, 2, 6, 8, 1, 3, 5, 7};
  
    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";
    }
    else if ( winner == human)
    {
        cout << winner << " 's won!\n";
    }
    else
    {
        cout << "It's a tie.\n";
    }
}