I wrote a python program utilizing the minimax algorithm to play tic tac toe. The board is represented by a 1d array. The player is able to play as either side. The program recursively generates all of the legal boards and checks for a winner alternating between minimizing and maximizing every turn. For some reason the algorithm never generates a winning board.
At first I assumed it was related to the algorithm not minimizing and maximizing for the right side but I checked and I don't see any reason the code wouldn't work
import random
def getOtherPlayer(letter):
if letter == "X":
return "O"
else:
return "X"
def getWinner(board):
for i in range(3):
# Check rows
if board[i*3] == board[i*3+1] == board[i*3+2] != "":
return board[i*3]
# Check columns
if board[i] == board[i+3] == board[i+6] != "":
return board[i]
# Check diagonals
if board[0] == board[4] == board[8] != "":
return board[0]
if board[2] == board[4] == board[6] != "":
return board[2]
# Check for tie
if "" not in board:
return "Tie"
# No winner yet
return None
def test():
board = ['X', '', 'O', 'X', 'O', 'X', 'O', 'X', 'O']
print(getScore(board, 'X', False))
def getAvailableMoves(board):
return [i for i in range(len(board)) if board[i] == ""]
def getScore(board, letter, isMaximizing):
winner = getWinner(board)
if winner == letter and isMaximizing:
print('win')
return 1 # current player wins
elif winner == letter and isMaximizing is False:
print('lose')
return -1
else:
print(board)
print(letter)
print(isMaximizing)
print('tie')
return 0 # tie (no winner)
def minimax(board, letter, player):
if player == 0:
if letter == "X":
bestScore = float('inf')
isMaximizing = False
else:
bestScore = float('-inf')
isMaximizing = True
if player == 1:
if letter == "O":
bestScore = float('inf')
isMaximizing = False
else:
bestScore = float('-inf')
isMaximizing = True
winner = getWinner(board)
if winner is not None:
print("terminal")
return getScore(board, letter, isMaximizing)
availableMoves = getAvailableMoves(board)
if isMaximizing:
for move in availableMoves:
board[move] = letter
score = minimax(board, getOtherPlayer(letter), player)
board[move] = ""
bestScore = max(score, bestScore)
else:
for move in availableMoves:
board[move] = letter
print(board)
score = minimax(board, getOtherPlayer(letter), player)
board[move] = ""
bestScore = min(score, bestScore)
return bestScore
def findBestMove(board, startingLetter, player):
availableMoves = getAvailableMoves(board)
bestMove = None
bestScore = -9999
for move in availableMoves:
board[move] = startingLetter
score = minimax(board, getOtherPlayer(startingLetter), player)
board[move] = ""
if score > bestScore:
bestScore = score
bestMove = move
return bestMove
def printBoard(board):
print()
for i in range(3):
print(board[i * 3:i * 3 + 3])
# Main Code
test()
letter = 'X'
board = [""] * 9
player = random.choice([0, 1])
print("You are playing as", player)
while True:
printBoard(board)
winner = getWinner(board)
if winner is not None:
if winner == "Tie":
print("Tie")
else:
print("Winner:", winner)
break
if player == 0:
if letter == "O":
move = findBestMove(board, letter, player)
print("Computer played move", move)
board[move] = letter
else:
move = int(input("Enter move (0-8): "))
while board[move] != "":
move = int(input("Invalid move. Enter move (0-8): "))
board[move] = letter
elif player == 1:
if letter == "X":
move = findBestMove(board, letter, player)
print("Computer played move", move)
board[move] = letter
else:
move = int(input("Enter move (0-8): "))
while board[move] != "":
move = int(input("Invalid move. Enter move (0-8): "))
board[move] = letter
letter = getOtherPlayer(letter)
