My code is for a Tic Tac Toe game and checking for a draw state but I think this question could be more useful in a general sense.
I have a list that represents the board, it looks like this:
board = [1,2,3,4,5,6,7,8,9]
When a player makes a move the int they moved on is replaced with their marker (‘x’ or ‘o’), I already have checks in place to look for a winning state, what I can’t do is check for a draw state, where none of the list values are ints but a winning state has not been set.
The code I have so far:
if any(board) != playerOne or any(board) != playerTwo:
print 'continue'
elif all(board) == playerOne or playerTwo:
print 'Draw'
The if statement works, the elif does not, I think the problem is my ‘or’ operator, what I want to check for is: if the every item on the board is either playerOne marker or playerTwo marker, if I where to make the code:
elif all(board) == playerOne or all(board) == playerTwo:
I would be checking to see if every place on the board was playerOne or every place on the board is playerTwo, which it won’t be.
So how do I check if the board is taken up by a combination of playerOne markers and playerTwo markers?
Answers:
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Method 1
Generally speaking:
all and any are functions that take some iterable and return True, if
- in the case of
all(), no values in the iterable are falsy; - in the case of
any(), at least one value is truthy.
A value x is falsy iff bool(x) == False.
A value x is truthy iff bool(x) == True.
Any non-booleans in the iterable will be fine — bool(x) will map (or coerce, if you prefer) any x according to these rules: 0, 0.0, None, [], (), [], set(), and other empty collections get mapped to False, anything else to True. The docstring for bool uses the terms ‘true’/’false’ for ‘truthy’/’falsy’, and True/False for the concrete boolean values.
In your specific code samples:
You misunderstood a little bit how these functions work. Hence, the following does something completely not what you thought:
if any(foobars) == big_foobar:
…because any(foobars) would first be evaluated to either True or False, and then that boolean value would be compared to big_foobar, which generally always gives you False (unless big_foobar coincidentally happened to be the same boolean value).
Note: the iterable can be a list, but it can also be a generator/generator expression (≈ lazily evaluated/generated list) or any other iterator.
What you want instead is:
if any(x == big_foobar for x in foobars):
which basically first constructs an iterable that yields a sequence of booleans—for each item in foobars, it compares the item to big_foobar and emits the resulting boolean into the resulting sequence:
tmp = (x == big_foobar for x in foobars)
then any walks over all items in tmp and returns True as soon as it finds the first truthy element. It’s as if you did the following:
In [1]: foobars = ['big', 'small', 'medium', 'nice', 'ugly'] In [2]: big_foobar = 'big' In [3]: any(['big' == big_foobar, 'small' == big_foobar, 'medium' == big_foobar, 'nice' == big_foobar, 'ugly' == big_foobar]) Out[3]: True
Note: As DSM pointed out, any(x == y for x in xs) is equivalent to y in xs but the latter is more readable, quicker to write and runs faster.
Some examples:
In [1]: any(x > 5 for x in range(4)) Out[1]: False In [2]: all(isinstance(x, int) for x in range(10)) Out[2]: True In [3]: any(x == 'Erik' for x in ['Erik', 'John', 'Jane', 'Jim']) Out[3]: True In [4]: all([True, True, True, False, True]) Out[4]: False
See also: http://docs.python.org/2/library/functions.html#all
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