Help with my Tic-Tac-Toe program

[thomas1991]

Hi, I'm currently working on a console application to play tic-tac-toe against the computer. I was able to get the computer to randomly select moves, but now I'm trying to make it smart. I created a function so the computer can defend any potential win, but when it attempts its first move, it gets stuck in an endless loop of invalid moves. Any suggestions on how I should change my function or how it is called when I want the computer to make a move?

Here's what the code looks like:

//Tic-Tac-Toe Human vs Computer

#include <iostream>
#include <stdlib.h>

using namespace std;

char sq[10] = {'z', '1', '2', '3', '4', '5', '6', '7', '8', '9'};
int choice;
int outcome ();
void board ();
int smart ();

int main ()
{
    int turn = 1, c;
    char mark;

    string welcome = "Welcome to Tic Tac Toe";

    do
    {
        cout << welcome << endl;
        cout << endl;

        board();
        if (turn % 2 == 1)
        {
            turn = 1;
        }
        else
        {
            turn = 2;
        }

        //human
        if (turn == 1)
        {
        cout << "Player " << turn << ", enter a number:" << endl;
        cin >> choice;
        }

        //computer
        else if (turn == 2)
        {
            choice = smart();
        }

        if (turn == 1)
        {
            mark = 'X';
        }
        else
        {
            mark = 'O';
        }

        if (choice == 1 && sq[1] == '1')
        {
            sq[1] = mark;
        }
        else if (choice == 2 && sq[2] == '2')
        {
            sq[2] = mark;
        }
        else if (choice == 3 && sq[3] == '3')
        {
            sq[3] = mark;
        }
        else if (choice == 4 && sq[4] == '4')
        {
            sq[4] = mark;
        }
        else if (choice == 5 && sq[5] == '5')
        {
            sq[5] = mark;
        }
        else if (choice == 6 && sq[6] == '6')
        {
            sq[6] = mark;
        }
        else if (choice == 7 && sq[7] == '7')
        {
            sq[7] = mark;
        }
        else if (choice == 8 && sq[8] == '8')
        {
            sq[8] = mark;
        }
        else if (choice == 9 && sq[9] == '9')
        {
            sq[9] = mark;
        }
        else
        {
            cout << "That number is taken. Choose another." << endl;
            turn --;
        }

        c = outcome();
        turn ++;

    } while (c == -1);

    board();

    if (c == 1)
    {
        cout << "Player " << --turn << " wins!" << endl;
    }
    else
    {
        cout << "This game is a draw" << endl;
    }
}

int smart ()       //function where the computer checks the board and makes a block if necessary
{
    if (sq[2] == sq[3])     //if squares 2 and 3 are the same
    {
        choice = 1;     //choose square 1 (follow this logic for each else if section)
    }
    else if (sq[4] == sq[7])
    {
        choice = 1;
    }
    else if (sq[5] == sq[9])
    {
        choice = 1;
    }
    else if (sq[1] == sq[3])
    {
        choice = 2;
    }
    else if (sq[5] == sq[8])
    {
        choice = 2;
    }
    else if (sq[1] == sq[2])
    {
        choice = 3;
    }
    else if (sq[6] == sq[9])
    {
        choice = 3;
    }
    else if (sq[5] == sq[7])
    {
        choice = 3;
    }
    else if (sq[1] == sq[7])
    {
        choice = 4;
    }
    else if (sq[5] == sq[6])
    {
        choice = 4;
    }
    else if (sq[4] == sq[6])
    {
        choice = 5;
    }
    else if (sq[2] == sq[8])
    {
        choice = 5;
    }
    else if (sq[1] == sq[9])
    {
        choice = 5;
    }
    else if (sq[3] == sq[7])
    {
        choice = 5;
    }
    else if (sq[4] == sq[5])
    {
        choice = 6;
    }
    else if (sq[3] == sq[9])
    {
        choice = 6;
    }
    else if (sq[1] == sq[4])
    {
        choice = 7;
    }
    else if (sq[8] == sq[9])
    {
        choice = 7;
    }
    else if (sq[3] == sq[5])
    {
        choice = 7;
    }
    else if (sq[7] == sq[9])
    {
        choice = 8;
    }
    else if (sq[2] == sq[5])
    {
        choice = 8;
    }
    else if (sq[7] == sq[8])
    {
        choice = 9;
    }
    else if (sq[3] == sq[6])
    {
        choice = 9;
    }
    else if (sq[1] == sq[5])
    {
        choice = 9;
    }
    else
    {
        choice = rand() % 9 + 1;
    }
    return 0;
}

int outcome ()      //function to determine the outcome of the game (win or draw)
{
    if (sq[1] == sq[2] && sq[2] == sq[3])       //if the mark on square 1 is equivalent to squares 2 and 3
    {
        return 1;                               //result is a win
    }
    else if (sq[4] == sq[5] && sq[5] == sq[6])
    {
        return 1;
    }
    else if (sq[7] == sq[8] && sq[8] == sq[9])
    {
        return 1;
    }
    else if (sq[1] == sq[4] && sq[4] == sq[7])
    {
        return 1;
    }
    else if (sq[2] == sq[5] && sq[5] == sq[8])
    {
        return 1;
    }
    else if (sq[3] == sq[6] && sq[6] == sq[9])
    {
        return 1;
    }
    else if (sq[1] == sq[5] && sq[5] == sq[9])
    {
        return 1;
    }
    else if (sq[3] == sq[5] && sq[5] == sq[7])
    {
        return 1;
    }
    else if (sq[1] != '1' && sq[2] != '2' && sq[3] != '3'
             && sq[4] != '4' && sq[5] != '5' && sq[6] != '6'
             && sq[7] != '7' && sq[8] != '8' && sq[9] != '9')
    {
        return 0;
    }
    else
    {
        return -1;
    }
}

void board ()       //function to display the game board
{
    cout << "Player 1 is X.    PLayer 2 is O." << endl;
    cout << endl;
    cout << "         |     |      " << endl;
    cout << "      " << sq[1] << "  |  " << sq[2] << "  |  " << sq[3] << endl;
    cout << "   ------|-----|------" << endl;
    cout << "      " << sq[4] << "  |  " << sq[5] << "  |  " << sq[6] << endl;
    cout << "   ------|-----|------" << endl;
    cout << "      " << sq[7] << "  |  " << sq[8] << "  |  " << sq[9] << endl;
    cout << "         |     |      " << endl;
    cout << endl;
}

[thomas1991]

here's an edit that made it somewhat work, but the computer gets stuck when I'm going to win with more than one combination of squares.

//Tic-Tac-Toe Human vs Computer by Thomas Fantasia

#include <iostream>
#include <stdlib.h>

using namespace std;

char sq[10] = {'z', '1', '2', '3', '4', '5', '6', '7', '8', '9'};
int choice;
int outcome ();
void board ();
int smart ();

int main ()
{
    int turn = 1, c;
    char mark;

    srand ( time(0));

    string welcome = "Welcome to Tic Tac Toe";

    do
    {
        cout << welcome << endl;
        cout << endl;

        board();
        if (turn % 2 == 1)
        {
            turn = 1;
        }
        else
        {
            turn = 2;
        }

        //human
        if (turn == 1)
        {
        cout << "Player " << turn << ", enter a number:" << endl;
        cin >> choice;
        }

        //computer
        else if (turn == 2)
        {
            if (sq[2] == sq[3])     //if squares 2 and 3 are the same
            {
                choice = 1;     //choose square 1 (follow this logic for each else if section)
            }
            else if (sq[4] == sq[7])
            {
                choice = 1;
            }
            else if (sq[5] == sq[9])
            {
                choice = 1;
            }
            else if (sq[1] == sq[3])
            {
                choice = 2;
            }
            else if (sq[5] == sq[8])
            {
                choice = 2;
            }
            else if (sq[1] == sq[2])
            {
                choice = 3;
            }
            else if (sq[6] == sq[9])
            {
                choice = 3;
            }
            else if (sq[5] == sq[7])
            {
                choice = 3;
            }
            else if (sq[1] == sq[7])
            {
                choice = 4;
            }
            else if (sq[5] == sq[6])
            {
                choice = 4;
            }
            else if (sq[4] == sq[6])
            {
                choice = 5;
            }
            else if (sq[2] == sq[8])
            {
                choice = 5;
            }
            else if (sq[1] == sq[9])
            {
                choice = 5;
            }
            else if (sq[3] == sq[7])
            {
                choice = 5;
            }
            else if (sq[4] == sq[5])
            {
                choice = 6;
            }
            else if (sq[3] == sq[9])
            {
                choice = 6;
            }
            else if (sq[1] == sq[4])
            {
                choice = 7;
            }
            else if (sq[8] == sq[9])
            {
                choice = 7;
            }
            else if (sq[3] == sq[5])
            {
                choice = 7;
            }
            else if (sq[7] == sq[9])
            {
                choice = 8;
            }
            else if (sq[2] == sq[5])
            {
                choice = 8;
            }
            else if (sq[7] == sq[8])
            {
                choice = 9;
            }
            else if (sq[3] == sq[6])
            {
                choice = 9;
            }
            else if (sq[1] == sq[5])
            {
                choice = 9;
            }
            else
            {
                choice = rand() % 9;
            }

        }

        if (turn == 1)
        {
            mark = 'X';
        }
        else
        {
            mark = 'O';
        }

        if (choice == 1 && sq[1] == '1')
        {
            sq[1] = mark;
        }
        else if (choice == 2 && sq[2] == '2')
        {
            sq[2] = mark;
        }
        else if (choice == 3 && sq[3] == '3')
        {
            sq[3] = mark;
        }
        else if (choice == 4 && sq[4] == '4')
        {
            sq[4] = mark;
        }
        else if (choice == 5 && sq[5] == '5')
        {
            sq[5] = mark;
        }
        else if (choice == 6 && sq[6] == '6')
        {
            sq[6] = mark;
        }
        else if (choice == 7 && sq[7] == '7')
        {
            sq[7] = mark;
        }
        else if (choice == 8 && sq[8] == '8')
        {
            sq[8] = mark;
        }
        else if (choice == 9 && sq[9] == '9')
        {
            sq[9] = mark;
        }
        else
        {
            cout << "That number is taken. Choose another." << endl;
            turn --;
        }

        c = outcome();
        turn ++;

    } while (c == -1);

    board();

    if (c == 1)
    {
        cout << "Player " << --turn << " wins!" << endl;
    }
    else
    {
        cout << "This game is a draw" << endl;
    }
}

int outcome ()      //function to determine the outcome of the game (win or draw)
{
    if (sq[1] == sq[2] && sq[2] == sq[3])       //if the mark on square 1 is equivalent to squares 2 and 3
    {
        return 1;                               //result is a win
    }
    else if (sq[4] == sq[5] && sq[5] == sq[6])
    {
        return 1;
    }
    else if (sq[7] == sq[8] && sq[8] == sq[9])
    {
        return 1;
    }
    else if (sq[1] == sq[4] && sq[4] == sq[7])
    {
        return 1;
    }
    else if (sq[2] == sq[5] && sq[5] == sq[8])
    {
        return 1;
    }
    else if (sq[3] == sq[6] && sq[6] == sq[9])
    {
        return 1;
    }
    else if (sq[1] == sq[5] && sq[5] == sq[9])
    {
        return 1;
    }
    else if (sq[3] == sq[5] && sq[5] == sq[7])
    {
        return 1;
    }
    else if (sq[1] != '1' && sq[2] != '2' && sq[3] != '3'
             && sq[4] != '4' && sq[5] != '5' && sq[6] != '6'
             && sq[7] != '7' && sq[8] != '8' && sq[9] != '9')
    {
        return 0;
    }
    else
    {
        return -1;
    }
}

void board ()       //function to display the game board
{
    cout << "Player 1 is X.    PLayer 2 is O." << endl;
    cout << endl;
    cout << "         |     |      " << endl;
    cout << "      " << sq[1] << "  |  " << sq[2] << "  |  " << sq[3] << endl;
    cout << "   ------|-----|------" << endl;
    cout << "      " << sq[4] << "  |  " << sq[5] << "  |  " << sq[6] << endl;
    cout << "   ------|-----|------" << endl;
    cout << "      " << sq[7] << "  |  " << sq[8] << "  |  " << sq[9] << endl;
    cout << "         |     |      " << endl;
    cout << endl;
}