Implementation of Tic-Tac-Toe game

Difficulty Level : Hard
Last Updated : 08 Feb, 2018

Rules of the Game

The game is to be played between two people (in this program between HUMAN and COMPUTER).
One of the player chooses ‘O’ and the other ‘X’ to mark their respective cells.
The game starts with one of the players and the game ends when one of the players has one whole row/ column/ diagonal filled with his/her respective character (‘O’ or ‘X’).

If no one wins, then the game is said to be draw.

Implementation
In our program the moves taken by the computer and the human are chosen randomly. We use rand() function for this.

What more can be done in the program?
The program is in not played optimally by both sides because the moves are chosen randomly. The program can be easily modified so that both players play optimally (which will fall under the category of Artificial Intelligence). Also the program can be modified such that the user himself gives the input (using scanf() or cin).
The above changes are left as an exercise to the readers.

Winning Strategy – An Interesting Fact
If both the players play optimally then it is destined that you will never lose (“although the match can still be drawn”). It doesn’t matter whether you play first or second.In another ways – “ Two expert players will always draw ”.
Isn’t this interesting ?

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

// A C++ Program to play tic-tac-toe
  
#include<bits/stdc++.h>
using namespace std;
  
#define COMPUTER 1
#define HUMAN 2
  
#define SIDE 3 // Length of the board
  
// Computer will move with 'O'
// and human with 'X'
#define COMPUTERMOVE 'O'
#define HUMANMOVE 'X'
  
// A function to show the current board status
void showBoard(char board[][SIDE])
{
    printf("\n\n");
      
    printf("\t\t\t  %c | %c  | %c  \n", board[0][0],
                             board[0][1], board[0][2]);
    printf("\t\t\t--------------\n");
    printf("\t\t\t  %c | %c  | %c  \n", board[1][0],
                             board[1][1], board[1][2]);
    printf("\t\t\t--------------\n");
    printf("\t\t\t  %c | %c  | %c  \n\n", board[2][0],
                             board[2][1], board[2][2]);
   
    return;
}
  
// A function to show the instructions
void showInstructions()
{
    printf("\t\t\t  Tic-Tac-Toe\n\n");
    printf("Choose a cell numbered from 1 to 9 as below"
            " and play\n\n");
      
    printf("\t\t\t  1 | 2  | 3  \n");
    printf("\t\t\t--------------\n");
    printf("\t\t\t  4 | 5  | 6  \n");
    printf("\t\t\t--------------\n");
    printf("\t\t\t  7 | 8  | 9  \n\n");
      
    printf("-\t-\t-\t-\t-\t-\t-\t-\t-\t-\n\n");
  
    return;
}
  
  
// A function to initialise the game 
void initialise(char board[][SIDE], int moves[])
{
    // Initiate the random number generator so that 
    // the same configuration doesn't arises
    srand(time(NULL)); 
      
    // Initially the board is empty
    for (int i=0; i<SIDE; i++)
    {
        for (int j=0; j<SIDE; j++)
            board[i][j] = ' ';
    }
      
    // Fill the moves with numbers
    for (int i=0; i<SIDE*SIDE; i++)
        moves[i] = i;
  
    // randomise the moves
    random_shuffle(moves, moves + SIDE*SIDE);
      
    return;
}
  
// A function to declare the winner of the game
void declareWinner(int whoseTurn)
{
    if (whoseTurn == COMPUTER)
        printf("COMPUTER has won\n");
    else
        printf("HUMAN has won\n");
    return;
}
  
// A function that returns true if any of the row
// is crossed with the same player's move
bool rowCrossed(char board[][SIDE])
{
    for (int i=0; i<SIDE; i++)
    {
        if (board[i][0] == board[i][1] &&
            board[i][1] == board[i][2] && 
            board[i][0] != ' ')
            return (true);
    }
    return(false);
}
  
// A function that returns true if any of the column
// is crossed with the same player's move
bool columnCrossed(char board[][SIDE])
{
    for (int i=0; i<SIDE; i++)
    {
        if (board[0][i] == board[1][i] &&
            board[1][i] == board[2][i] && 
            board[0][i] != ' ')
            return (true);
    }
    return(false);
}
  
// A function that returns true if any of the diagonal
// is crossed with the same player's move
bool diagonalCrossed(char board[][SIDE])
{
    if (board[0][0] == board[1][1] &&
        board[1][1] == board[2][2] && 
        board[0][0] != ' ')
        return(true);
          
    if (board[0][2] == board[1][1] &&
        board[1][1] == board[2][0] &&
         board[0][2] != ' ')
        return(true);
  
    return(false);
}
  
// A function that returns true if the game is over
// else it returns a false
bool gameOver(char board[][SIDE])
{
    return(rowCrossed(board) || columnCrossed(board)
            || diagonalCrossed(board) );
}
  
// A function to play Tic-Tac-Toe
void playTicTacToe(int whoseTurn)
{
    // A 3*3 Tic-Tac-Toe board for playing 
    char board[SIDE][SIDE];
      
    int moves[SIDE*SIDE];
      
    // Initialise the game
    initialise(board, moves);
      
    // Show the instructions before playing
    showInstructions();
      
    int moveIndex = 0, x, y;
      
    // Keep playing till the game is over or it is a draw
    while (gameOver(board) == false && 
            moveIndex != SIDE*SIDE)
    {
        if (whoseTurn == COMPUTER)
        {
            x = moves[moveIndex] / SIDE;
            y = moves[moveIndex] % SIDE;
            board[x][y] = COMPUTERMOVE;
            printf("COMPUTER has put a %c in cell %d\n",
                    COMPUTERMOVE, moves[moveIndex]+1);
            showBoard(board);
            moveIndex ++;
            whoseTurn = HUMAN;
        }
          
        else if (whoseTurn == HUMAN)
        {
            x = moves[moveIndex] / SIDE;
            y = moves[moveIndex] % SIDE;
            board[x][y] = HUMANMOVE;
            printf ("HUMAN has put a %c in cell %d\n",
                    HUMANMOVE, moves[moveIndex]+1);
            showBoard(board);
            moveIndex ++;
            whoseTurn = COMPUTER;
        }
    }
  
    // If the game has drawn
    if (gameOver(board) == false && 
            moveIndex == SIDE * SIDE)
        printf("It's a draw\n");
    else
    {
        // Toggling the user to declare the actual
        // winner
        if (whoseTurn == COMPUTER)
            whoseTurn = HUMAN;
        else if (whoseTurn == HUMAN)
            whoseTurn = COMPUTER;
          
        // Declare the winner
        declareWinner(whoseTurn);
    }
    return;
}
  
// Driver program
int main()
{
    // Let us play the game with COMPUTER starting first
    playTicTacToe(COMPUTER);
      
    return (0);
}

Output:

                Tic-Tac-Toe

Choose a cell numbered from 1 to 9 as below and play

              1 | 2  | 3  
            --------------
              4 | 5  | 6  
            --------------
              7 | 8  | 9  

-    -    -    -    -    -    -    -    -    -

COMPUTER has put a O in cell 6


                |    |    
            --------------
                |    | O  
            --------------
                |    |    

HUMAN has put a X in cell 7


                |    |    
            --------------
                |    | O  
            --------------
              X |    |    

COMPUTER has put a O in cell 5


                |    |    
            --------------
                | O  | O  
            --------------
              X |    |    

HUMAN has put a X in cell 1


              X |    |    
            --------------
                | O  | O  
            --------------
              X |    |    

COMPUTER has put a O in cell 9


              X |    |    
            --------------
                | O  | O  
            --------------
              X |    | O  

HUMAN has put a X in cell 8


              X |    |    
            --------------
                | O  | O  
            --------------
              X | X  | O  

COMPUTER has put a O in cell 4


              X |    |    
            --------------
              O | O  | O  
            --------------
              X | X  | O  

COMPUTER has won

An Interesting Variant of this game
As said above, if two experienced players are playing the Tic-Tac-Toe, then the game will always draw.
There is another viral variant of this game- Ultimate Tic-Tac-Toe, which aims to make the normal Tic-Tac-Toe more interesting and less predictable.
Have a look at the game here- Link1 Link2

The above article implements simple Tic-Tac-Toe where moves are randomly made. Please refer below article to see how optimal moves are made.
Minimax Algorithm in Game Theory | Set 3 (Tic-Tac-Toe AI – Finding optimal move)

Great discussions on the “winning/never losing” strategy
Quora
Wikihow

This article is contributed by Rachit Belwariar .If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

My Personal Notes

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Recommended: Please try your approach on {IDE} first, before moving on to the solution.