Write a Tic-Tac-Toe program that allows two people to play the game against each other. In turn, ask each player which row and column they want to play. Make sure that the program checks if that row/column combination is empty. When a player has won, end the game. When the whole board is full and there is no winner, announce a draw.

This is a fairly long program to write (60 lines or so). It will definitely help to use some functions. I recommend that you create a function display\_board() that gets the board as parameter and displays it, a function getRowCol() that asks for a row or a column (depending on a parameter) and checks whether the user entered a legal value, and a function winner() that gets the board as argument and checks if there is a winner. Keep track of who the current player is using a global variable player that you can pass to a function as an argument if the function needs it. I also use a function opponent(), that takes the player as argument and returns the opponent. I use that to switch players after each move.

The main program will be something along the lines of (in pseudo-code):

display board
while True:
    ask for row
    ask for column
    if row/column combination already occupied:
        display error message
        continue
    place player marker on row/column combination
    display board
    if there is a winner:
        announce winner
        break
    if the board is full:
        announce draw
        break
    switch players

Assignment

Represent the board as a list (list) of three rows, where each row is a list (list) of three strings (str): X for a cell taken by the first player, O for a cell taken by the second player, and . for an empty cell. Rows and columns are numbered 1 up to and including 3. Write the following five functions, and use them to write the main program sketched above.

• Write a function opponent that takes a player (X or O). The function must return the other player.

• Write a function display_board that takes a board. The function must print the board, preceded by a line that holds the column numbers, and with every row preceded by its row number. Separate the cells of a row by a single space.

• Write a function place that takes a board, a player, a row and a column. If the given row and column are on the board and the corresponding cell is still empty, then the function must place the marker of the given player in that cell and return True. Otherwise, the function must leave the board untouched and return False.

• Write a function winner that takes a board. If a player has three markers on a row, on a column or on a diagonal, then the function must return that player. Otherwise, the function must return None.

• Write a function full that takes a board. The function must return a Boolean value (bool) that indicates whether all cells of the board are taken.

Example

>>> opponent('X')
'O'
>>> board = [['.', '.', '.'], ['.', '.', '.'], ['.', '.', '.']]
>>> place(board, 'X', 2, 2)
True
>>> place(board, 'O', 2, 2)
False
>>> place(board, 'O', 4, 1)
False
>>> place(board, 'O', 1, 3)
True
>>> display_board(board)
  1 2 3
1 . . O
2 . X .
3 . . .
>>> winner(board)
>>> full(board)
False
>>> display_board([['X', 'O', 'O'], ['.', 'X', '.'], ['O', '.', 'X']])
  1 2 3
1 X O O
2 . X .
3 O . X
>>> winner([['X', 'O', 'O'], ['.', 'X', '.'], ['O', '.', 'X']])
'X'
>>> full([['X', 'O', 'X'], ['X', 'O', 'O'], ['O', 'X', 'X']])
True