Python two-dimensional array – changing an element [closed]

I’m gonna take a stab at this one even though the behavior you describe (as you’ve described it) isn’t possible.

If you create a list, you need to make sure that each sublist is a different list. Consider:

a = []
b = [a, a]

Here I’ve created a list where both of the sublists are the exact same list. If I change one, it will show up in both. e.g.:

>>> a = []
>>> b = [a, a]
>>> b[0].append(1)
>>> b
[[1], [1]]

you’ll frequently see this behavior with a list initialized using the * operator:

a = [[None]*7]*7

e.g.

>>> a = [[None]*7]*7
>>> a
[[None, None, None, None, None, None, None], [None, None, None, None, None, None, None], [None, None, None, None, None, None, None], [None, None, None, None, None, None, None], [None, None, None, None, None, None, None], [None, None, None, None, None, None, None], [None, None, None, None, None, None, None]]
>>> a[0][1] = 3
>>> a
[[None, 3, None, None, None, None, None], [None, 3, None, None, None, None, None], [None, 3, None, None, None, None, None], [None, 3, None, None, None, None, None], [None, 3, None, None, None, None, None], [None, 3, None, None, None, None, None], [None, 3, None, None, None, None, None]]

The fix is to not use the * 7 on the outer list (the inner list is OK since None is immutable):

a = [[None]*7 for _ in range(7)]

e.g.:

>>> a = [[None]*7 for _ in range(7)]
>>> a[0][1] = 3
>>> a
[[None, 3, None, None, None, None, None], [None, None, None, None, None, None, None], [None, None, None, None, None, None, None], [None, None, None, None, None, None, None], [None, None, None, None, None, None, None], [None, None, None, None, None, None, None], [None, None, None, None, None, None, None]]