Python tic tac toe detecting cheating

Question

So for now I have written a simple Tic tac toe game. I haven't even used all the functions I made but here is my question: How can I determine if the player places a 1 or 2 where there already is one, I think I know how to do this, but how can I then put them back at the "Input your number" prompt if they enter an illegal character or they try to overwrite an already placed 1 or 2.

Also is there a more compact way to do this?

Here is the code for the game:

nr = [0,0,0,0,0,0,0,0,0]
keepGoing = True

def checkP1():
    if nr[0] and nr[1] and nr[2] or nr[3] and nr[4] and nr[5] or nr[6] and nr[7] and nr[8] or nr[0] and nr[3] and \
       nr[6] or nr[1] and nr[4] and nr[7] or nr[2] and nr[5] and nr[8] or nr[0] and nr[4] and nr[8] or nr[2] and nr[4] and nr[6] == 1:
        print("P1 Wins")
        keepGoing = False
        return keepGoing



def checkP2():
    if nr[0] and nr[1] and nr[2] or nr[3] and nr[4] and nr[5] or nr[6] and nr[7] and nr[8] or nr[0] and nr[3] and \
       nr[6] or nr[1] and nr[4] and nr[7] or nr[2] and nr[5] and nr[8] or nr[0] and nr[4] and nr[8] or nr[2] and nr[4] and nr[6] == 2:
        print("P2 Wins")
        keepGoing = False
        return keepGoing

def Game():
    while keepGoing:
        PrintBoard()
        in1 = 0
        in2 = 0
        in1 = input("Please enter the number of the position you want to put your symbol P1.")
        nr[int(in1)-1] = 1
        check = checkP1()
        if check == 0:
            PrintBoard()
            break
        in2 = input("Please enter the number of the position you want to put your symbol P2.")
        check = checkP2()
        if check == 0:
            PrintBoard()
            break
        nr[int(in2)-1] = 2

def PrintBoard():
    print("",nr[0],nr[1],nr[2],"\n",nr[3],nr[4],nr[5],"\n",nr[6],nr[7],nr[8])

def Reset():
    nr = [0,0,0,0,0,0,0,0,0]

    keepGoing = True

You should at least use `all()` to test for a winner, your `if` clauses are a mess. column Y: `all(nr[Y::3])`, row X: `all(nr[X:X+3])`, diagonals: `all(nr[::4])` and `all(nr[2:8:2])` — CodeManX, Apr 25 '14 at 12:10
@CoDEmanX I forgot to state I am beginner, I don't really know a lot yet. I was trying what I can do with my current resources. — Czarek, Apr 25 '14 at 12:29
@CoDEmanX I actually stopped learning for a while. Now when I look at this I get your point. I know that lists can take 2 arguments like list[x:y] but what happens if you give them 3 arguments like you did? list[x:y:z] What does it mean? — Czarek, May 27 '14 at 13:58
The third argument is the step value, so you can read [2:8:2] as "start at 2, count up to 8 (exclusive) in steps of 2". `list(range(9))[2:8:2]` or `[0, 1, 2, 3, 4, 5, 6, 7, 8][2:8:2]` returns `[2, 4, 6]`. `[::4]` means "return every 4th element", which is `[0, 4, 8]` here. https://docs.python.org/3.5/library/functions.html#slice — CodeManX, May 27 '14 at 14:28
@CoDEmanX I get it, the thing is I don't see a way that I could use this to make it more compact. I understand the way that it should work I just don't see a way to do it in code. It would be awesome if you could rewrite my check statements and show me what they should look like. — Czarek, May 27 '14 at 16:40

score 1 · Answer 1 · edited May 23 '17 at 10:32

To answer your specific question

how can I then put them back at the "Input your number" prompt if they enter an illegal character or they try to overwrite an already placed 1 or 2

I would make a function to do this:

def get_valid_input(board):
    while True:
        try:
            move = int(input("Please enter the number of the position you want to put your symbol."))
        except ValueError:
            print("Input must be an integer number.")
        else:
            if move not in range(1, 10):
                print("Move must be 1-9.")
            elif board[move-1] in (1, 2):
                print("Location already used.")
            else:
                return move

This will continue until whichever player is currently going gives a valid move:

in1 = get_valid_input(nr)

A few more general pointers:

Your current checking for a winner doesn't work - if a or b == c doesn't do what you think it does (see e.g. this question). For example, nr[0] and nr[1] and nr[2] == 2 actually tests bool(nr[0]) and bool(nr[1]) and (nr[2] == 2); as long as the last value is two and the other two aren't zero, it will be True. CoDEmanX's suggestions in the comments will be useful here.
Using the flag keepgoing isn't very Pythonic; I would make a function game_over(board) that returns True if the game is finished (either a win or a draw) and False otherwise, then the overall loop becomes while True: ... if game_over(board): break.
Rather than relying on scope to access variables, pass what you need explicitly (e.g. the board argument to get_valid_input and game_over). Getting rid of keepgoing removes one of your global variables, but you could take the board as an argument to you other functions and return as appropriate. Alternatively, consider making a class to hold the board and all the functions.

I really don't understand your 2 first bulleted points. I don't know if that is bad but I am just a beginner in python. As your code makes complete sense I don't really understand what the text below it means. — Czarek, Apr 26 '14 at 06:09
I have added more to the comments. Have you read the linked question? If you don't understand the terms, try a web search for them. — jonrsharpe, Apr 26 '14 at 07:01

score 0 · Accepted Answer · answered May 27 '14 at 22:23

An example of how slices can be used to determine the winner:

def same(iterable):
    it = iter(iterable)
    start_val = it.__next__()
    for val in it:
        if val != start_val:
            return
    if start_val != 0:
        return start_val

nr = [0] * 9

nr[1] = 1
nr[2] = 2
nr[4] = 2
nr[6] = 2
nr[7] = 1
nr[8] = 1

for i in range(0, len(nr), 3):
    print(nr[i:i+3])

winner = (
    same(nr[0:3]) or same(nr[3:6]) or same(nr[6:9]) or
    same(nr[0::3]) or same(nr[1::3]) or same(nr[2::3]) or
    same(nr[::4]) or same(nr[2:-1:2])
)

if winner is not None:
    print("Player %i won!" % winner)

same() is a helper function that checks if all elements of an iterable have the same value, and if so, return that value unless it is 0 (so we only catch 1s and 2s by players).

You see the patterns in the slice notations, they could be easily scaled by using for-loops.

Python tic tac toe detecting cheating

2 Answers2