52 — Game: tic-tac-toe

Guide 51 built a board; guide 26 showed how ask, while True, and break make a game loop. Tic-tac-toe joins them: two players take turns picking a row and a column, and after every move a function checks for three in a row.

#Steps

1. The whole game in one file, tictactoe.nme. The board and show_board come from 51; check_winner scans rows, columns, and both diagonals and returns the winning mark, or None while no line is complete; ask number reads a row and a column as integers, continue re-asks a taken square, and moves == 9 declares a draw:

   # tictactoe.nme — a two-player tic-tac-toe game.
   # Run: nme r tictactoe
   # The board is a list of rows (guide 51). A turn places a mark,
   # and check_winner looks for three in a row, column, or diagonal.
   
   board = [["-", "-", "-"], ["-", "-", "-"], ["-", "-", "-"]]
   
   def show_board(board):
       for row in board:
           show row
   
   def check_winner(board):
       # Rows: three equal marks side by side.
       for row in board:
           if row[0] == row[1] and row[1] == row[2] and row[0] != "-":
               return row[0]
       # Columns: three equal marks straight down.
       for col in range(3):
           if board[0][col] == board[1][col] and board[1][col] == board[2][col] and board[0][col] != "-":
               return board[0][col]
       # Diagonals: corner to corner.
       if board[0][0] == board[1][1] and board[1][1] == board[2][2] and board[0][0] != "-":
           return board[0][0]
       if board[0][2] == board[1][1] and board[1][1] == board[2][0] and board[0][2] != "-":
           return board[0][2]
       return None
   
   player = "X"
   moves = 0
   show "Tic-tac-toe! Rows and columns are 0, 1, or 2."
   show "Line up three marks to win. X goes first."
   
   while True:
       show_board(board)
       ask number row, f"Player {player}, row (0-2): "
       ask number col, f"Player {player}, column (0-2): "
       if board[row][col] != "-":
           show "That square is taken. Pick another."
           continue
       board[row][col] = player
       moves = moves + 1
       winner = check_winner(board)
       if winner is not None:
           show f"After {moves} moves:"
           show_board(board)
           show f"Player {winner} wins!"
           break
       if moves == 9:
           show f"After {moves} moves:"
           show_board(board)
           show "It is a draw."
           break
       if player == "X":
           player = "O"
       else:
           player = "X"
   

   copyrun it →
2. Run a whole game through a pipe — row then column for five moves. X fills a row and wins on the last move:

   printf '0\n0\n1\n0\n0\n1\n2\n0\n0\n2\n' | nme r tictactoe
   

   copy

   Tic-tac-toe! Rows and columns are 0, 1, or 2.
   Line up three marks to win. X goes first.
   ['-', '-', '-']
   ['-', '-', '-']
   ['-', '-', '-']
   Player X, row (0-2): Player X, column (0-2): ['X', '-', '-']
   ['-', '-', '-']
   ['-', '-', '-']
   Player O, row (0-2): Player O, column (0-2): ['X', '-', '-']
   ['O', '-', '-']
   ['-', '-', '-']
   Player X, row (0-2): Player X, column (0-2): ['X', 'X', '-']
   ['O', '-', '-']
   ['-', '-', '-']
   Player O, row (0-2): Player O, column (0-2): ['X', 'X', '-']
   ['O', '-', '-']
   ['O', '-', '-']
   Player X, row (0-2): Player X, column (0-2): After 5 moves:
   ['X', 'X', 'X']
   ['O', '-', '-']
   ['O', '-', '-']
   Player X wins!
   

   copy

   The last board shows the winning row: X across the top. The game ends with break, so the winning move is the last move.

#Try it yourself

Play a game O wins and one that ends in a draw, then make check_winner also name the winning line, such as "row 0".

#What you learned

• ask number reads a row and column as integers; board[row][col] writes a mark.
• A function checks rows, columns, and diagonals for three equal marks.
• continue rejects a taken square; break ends the game.
• moves == 9 with no winner declares a draw.