Error in repeating input in a tictactoe game

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I am trying to prevent the user from inputting a square that is already marked, but the for loop moves on to the next player's input without decrementing the value of i by one, so the player 1 can repeat his input. How do I fix this?


arr = [[0,0,0],[0,0,0],[0,0,0]]
grid = grid(arr)
grid.print_grid()

for i in range(9):
    row = int(input("Enter the row name: "))
    col = int(input("Enter the column name: "))
    if(arr[row][col] == 0):
        if(i%2):
            arr[row][col] = 1
        else:
            arr[row][col] = 2
    else:
        print("\nThat square has already been marked! Please select another square")
        i = i-1
        continue
    grid.print_grid()
    res = grid.grid_checker()
    if (res == 1):
        print("\nPlayer 1 wins the game!")
        break
    elif(res == 2):
        print("\nPlayer 2 wins the game!")
        break
    elif(i == 8):
        print("\nThe game has ended in a draw!")
continue python loops
1 Answers

You need to store another variable to keep track of whose turn it is. You cannot modify the variable you are looping on while you are in the loop body. This means that i cannot be manipulated while you are running in the loop. Here's how I would change it.

turn = 0
while True:
    row = int(input("Enter the row name: "))
    col = int(input("Enter the column name: "))
    if(arr[row][col] == 0):
        if(i%2):
            arr[row][col] = 1
            turn = turn + 1
        else:
            arr[row][col] = 2
            turn = turn + 1
    else:
        print("\nThat square has already been marked! Please select another square")
        continue
    grid.print_grid()
    res = grid.grid_checker()
    if (res == 1):
        print("\nPlayer 1 wins the game!")
        break
    elif(res == 2):
        print("\nPlayer 2 wins the game!")
        break
    elif(turn == 8):
        print("\nThe game has ended in a draw!")

Here we're saving the turn number in the variable turn and only incrementing the variable when we can confirm a player has successfully completed their turn.

Why you cannot modify i: For optimisations, loops are often expanded by python before they are converted to assembly instructions. For example a simple loop like this:

for i in range(9):
    print(i)

May be expanded to something like this:

i = 0
print(i)
i = 1
print(i)
# and so on, upto
i = 9
print(i)

This is done to avoid having to jump around in memory. So as you can see here, i is reassigned for every iteration of the loop. Therefore, even if you change the value of i in the body of the loop, it will simply be reassigned before the next iteration.

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