Creating the game.

Creating a Board

Like I mentioned above we will be using a grid system to represent our board. To do this in python we will create a list called board that will start off with 10 empty values. The reason we have 10 empty values rather than 9 is because when we get input from the user they can type numbers 1-9 NOT 0-8. So to make our lives easier we are going make the first value of our list an empty string. This way when we index elements in our list we can use 1-9 not 0-8.

board = [' ' for x in range(10)]  # This should be the first line in your program
# board is now: [' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ']

insertLetter()

This function is going to take two parameters: letter & pos. It is simply going to insert the given letter at the given position.

def insertLetter(letter, pos):
    board[pos] = letter

spaceIsFree(pos)

This function will simply tell us if the given space is free. Meaning it does not already contain a letter. It has one parameter, pos, which will be an integer from 1-9.

def spaceIsFree(pos):
    return board[pos] == ' '

# This function will return a True or False value

printBoard(board)

This function takes the board as a parameter and will display it to the console.

def printBoard(board):
    # "board" is a list of 10 strings representing the board (ignore index 0)
    print('   |   |')
    print(' ' + board[1] + ' | ' + board[2] + ' | ' + board[3])
    print('   |   |')
    print('-----------')
    print('   |   |')
    print(' ' + board[4] + ' | ' + board[5] + ' | ' + board[6])
    print('   |   |')
    print('-----------')
    print('   |   |')
    print(' ' + board[7] + ' | ' + board[8] + ' | ' + board[9])
    print('   |   |')

isWinner()

This function will tell us if the given letter has won based on the current board. It has two parameters: bo(board) & le(letter). The letter must be a “X” or an “O”. We will simply check each possible winning line on the board and see if it is populated by the given letter.

def isWinner(bo, le):
    # Given a board and a player’s letter, this function returns True if that player has won.
    # We use bo instead of board and le instead of letter so we don’t have to type as much.
    return ((bo[7] == le and bo[8] == le and bo[9] == le) or # across the top
    (bo[4] == le and bo[5] == le and bo[6] == le) or # across the middle
    (bo[1] == le and bo[2] == le and bo[3] == le) or # across the bottom
    (bo[7] == le and bo[4] == le and bo[1] == le) or # down the left side
    (bo[8] == le and bo[5] == le and bo[2] == le) or # down the middle
    (bo[9] == le and bo[6] == le and bo[3] == le) or # down the right side
    (bo[7] == le and bo[5] == le and bo[3] == le) or # diagonal
    (bo[9] == le and bo[5] == le and bo[1] == le)) # diagonal

main()

This function is what we will call to start the game. It will be calling all of the different functions in our program and dictate the flow of the program.

def main():
    #Main game loop
    print('Welcome to Tic Tac Toe, to win complete a straight line of your letter (Diagonal, Horizontal, Vertical). The board has positions 1-9 starting at the top left.')
    printBoard()

    while not(isBoardFull(board)):
        if not(isWinner(board, 'O')):
            playerMove()
            printBoard()
        else:
            print('O\'s win this time...')
            break

        
        if not(isWinner(board, 'X')):
            move = compMove()
            if move == 0:
                print('Game is a Tie! No more spaces left to move.')
            else:
                insertBoard('O', move)
                print('Computer placed an \'O\' in position', move, ':')
                printBoard()
        else:
            print('X\'s win, good job!')
            break

    if isBoardFull(board):
        print('Game is a tie! No more spaces left to move.')

isBoardFull()

This function takes board as parameter and will simply return True if the board is full and False if it is not.

def isBoardFull(board):
    if board.count(' ') > 1:  # Since we always have one blank element in board we must use > 1
        return False
    else:
        return True

playerMove()

In this function we will be asking the user to input a move and validating it. If the move is valid we will add that letter to the board. Otherwise we will continue to ask the user for input.

def playerMove():
    run = True
    while run:  # Keep looping until we get a valid move
        move = input('Please select a position to place an \'X\' (1-9): ')
        try:
            move  = int(move)
            if move > 0 and move < 10:  # makes sure we type in a number between 1-9
                if spaceIsFree(move):  # check if the move we choose is valid (no other letter is there already)
                    run = False
                    insertBoard('X', move)
                else:
                    print('This postion is already occupied!')
            else:
                print('Please type a number within the range!')
        except:
            print('Please type a number!')

compMove()

Now time for the AI! This function will be responsible for making the computers move. It will examine the board and determine which move is the best to make. The algorithm we will follow to do this is listed below.

If the current step cannot be completed proceed to the next.

1. If there is a winning move take it.
2. If the player has a possible winning move on their next turn move into that position.
3. Take any one of the corners. If more than one is available randomly decide.
4. Take the center position.
5. Take one of the edges. If more than one is available randomly decide.
6. If no move is possible the game is a tie.

def compMove():
    possibleMoves = [x for x, letter in enumerate(board) if letter == ' ' and x != 0] # Create a list of possible moves
    move = 0
    
    #Check for possible winning move to take or to block opponents winning move
    for let in ['O','X']:
        for i in possibleMoves:
            boardCopy = board[:]
            boardCopy[i] = let
            if isWinner(boardCopy, let):
                move = i
                return move


    #Try to take one of the corners
    cornersOpen = []
    for i in possibleMoves:
        if i in [1,3,7,9]:
            cornersOpen.append(i)
    if len(cornersOpen) > 0:
        move = selectRandom(cornersOpen)
        return move
    
    #Try to take the center
    if 5 in possibleMoves:
        move = 5
        return move

    #Take any edge
    edgesOpen = []
    for i in possibleMoves:
        if i in [2,4,6,8]:
            edgesOpen.append(i)
    
    if len(edgesOpen) > 0:
        move = selectRandom(edgesOpen)

    return move

selectRandom()

This function will randomly decide on a move to take given a list of possible positions.

def selectRandom(li):
    import random
    ln = len(li)
    r = random.randrange(0, ln)
    return li[r]

Starting the Game

Now that we have all our functions completed all that’s left to do is start the game. If we just wanted to run the game once all we would have to do is call main. However, in our case we want the game to keep running until the user doesn’t want to play anymore, so we will create a small while loop in the main line.

while True:
    answer = input('Do you want to play again? (Y/N)')
    if answer.lower() == 'y' or answer.lower == 'yes':
        board = [' ' for x in range(10)]
        print('-----------------------------------')
        main()
    else:
        break

Full Code

Now that our game is finished here is the full code.

#Tic Tac Toe game in python

board = [' ' for x in range(10)]

def insertLetter(letter, pos):
    board[pos] = letter

def spaceIsFree(pos):
    return board[pos] == ' '

def printBoard(board):
    print('   |   |')
    print(' ' + board[1] + ' | ' + board[2] + ' | ' + board[3])
    print('   |   |')
    print('-----------')
    print('   |   |')
    print(' ' + board[4] + ' | ' + board[5] + ' | ' + board[6])
    print('   |   |')
    print('-----------')
    print('   |   |')
    print(' ' + board[7] + ' | ' + board[8] + ' | ' + board[9])
    print('   |   |')
    
def isWinner(bo, le):
    return (bo[7] == le and bo[8] == le and bo[9] == le) or (bo[4] == le and bo[5] == le and bo[6] == le) or(bo[1] == le and bo[2] == le and bo[3] == le) or(bo[1] == le and bo[4] == le and bo[7] == le) or(bo[2] == le and bo[5] == le and bo[8] == le) or(bo[3] == le and bo[6] == le and bo[9] == le) or(bo[1] == le and bo[5] == le and bo[9] == le) or(bo[3] == le and bo[5] == le and bo[7] == le)

def playerMove():
    run = True
    while run:
        move = input('Please select a position to place an \'X\' (1-9): ')
        try:
            move = int(move)
            if move > 0 and move < 10:
                if spaceIsFree(move):
                    run = False
                    insertLetter('X', move)
                else:
                    print('Sorry, this space is occupied!')
            else:
                print('Please type a number within the range!')
        except:
            print('Please type a number!')
            

def compMove():
    possibleMoves = [x for x, letter in enumerate(board) if letter == ' ' and x != 0]
    move = 0

    for let in ['O', 'X']:
        for i in possibleMoves:
            boardCopy = board[:]
            boardCopy[i] = let
            if isWinner(boardCopy, let):
                move = i
                return move

    cornersOpen = []
    for i in possibleMoves:
        if i in [1,3,7,9]:
            cornersOpen.append(i)
            
    if len(cornersOpen) > 0:
        move = selectRandom(cornersOpen)
        return move

    if 5 in possibleMoves:
        move = 5
        return move

    edgesOpen = []
    for i in possibleMoves:
        if i in [2,4,6,8]:
            edgesOpen.append(i)
            
    if len(edgesOpen) > 0:
        move = selectRandom(edgesOpen)
        
    return move

def selectRandom(li):
    import random
    ln = len(li)
    r = random.randrange(0,ln)
    return li[r]
    

def isBoardFull(board):
    if board.count(' ') > 1:
        return False
    else:
        return True

def main():
    print('Welcome to Tic Tac Toe!')
    printBoard(board)

    while not(isBoardFull(board)):
        if not(isWinner(board, 'O')):
            playerMove()
            printBoard(board)
        else:
            print('Sorry, O\'s won this time!')
            break

        if not(isWinner(board, 'X')):
            move = compMove()
            if move == 0:
                print('Tie Game!')
            else:
                insertLetter('O', move)
                print('Computer placed an \'O\' in position', move , ':')
                printBoard(board)
        else:
            print('X\'s won this time! Good Job!')
            break

    if isBoardFull(board):
        print('Tie Game!')

while True:
    answer = input('Do you want to play again? (Y/N)')
    if answer.lower() == 'y' or answer.lower == 'yes':
        board = [' ' for x in range(10)]
        print('-----------------------------------')
        main()
    else:
        break

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