Initializing and updating a multidemensional array python3

Question

I need to initialize a multidimensional array from an integer n.

n = 2

list_1 = [[] for _ in range(n)]
list_2 = [[]] * n

Then I append 2 to the first item in the nD-array as below.

list_1[0].append(2)
list_2[0].append(2)

print('After')
print(list_1) # [[2], []]
print(list_2) # [[2], [2]] Why this happen?

You could find the code here.

I'm new to python. Could anyone explain in-depth why when I use [[]] *n, it will update all child arrays in list_2?

From what I can tell this is because in list_1 every item is uniquely created but in list_2 all the items are duplicates of each other. So, whatever happens to one item in list_2 will happen to the rest. — Guac
– Guac, Commented Dec 21, 2020 at 8:54

Viet · Accepted Answer · 2020-12-21 09:07:06Z

1

You can find a comprehensive answer here: Python list confusion

Basically when the array is initialized with *n, it will copy the address of the first element to all the rest.

answered Dec 21, 2020 at 9:07

Viet

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