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+--- Thread: 2-D list element assignment (/thread-10552.html)
2-D list element assignment - ashutosh759 - May-24-2018
nm = 4 c = [[0, False]] * nm c[0][0] = 1 print(c)I expected that it should give an output as [0, False] (Printed 4 times). Instead, the output is [1, False](Printed 4 times).
Can anyone explain?
RE: 2-D list element assignment - scidam - May-25-2018
[x]*n -- replicates x n-times, i.e. [x]*n = [x, x, ....ntimes, x]
[0, False] is a mutable object (it is a list), so, each time it is replicated, internally, Python uses a pointer to the same object: so a target list -- [x, x, ..., x] is just a set of the same objects.
So, when you change one of them, you change all object(s) at once.
You can avoid this behavior if you define the list as follows:
z = [[0, False], [0, False], [0, False], [0, False]]In this case,
[0, False] items are internally presented as different objects, so, if you try to change one of them, e.g.
z[0][0] = 1, this willn't affect on others.