CODEWITHSUNDEEP
python-3.xlist-comprehension
helloworld
helloworld

Reputation: 169

why list index has different result?

Below is my code.

check = [[0] * 4] * 2
check[0][1] = 1
print(check)
check2 = [[0, 0, 0, 0], [0, 0, 0,0]]
check2[0][1] = 1
print(check2)

result:

[[0, 1, 0, 0], [0, 1, 0, 0]]
[[0, 1, 0, 0], [0, 0, 0, 0]]

I expect check and check2 get same result but it is different.

why check[0][1] and check[1][1] are changed?? not only check[0][0]

Upvotes: 0

Views: 30

Answers (1)

Ram
Ram

Reputation: 4779

check is a shallow copy

check = [[0] * 4] * 2 - Creates only one list [0,0,0,0] and rows 1 and 2 refer to this list.

So changing the elements of one row reflects changes in every other row.

To avoid such a scenario you can use a for-loop to create the list like this

check = [[0 for _ in range(4)] for _ in range(2)]

Now, this will be a deep copy and not shallow copy.

check2 is a deep copy.

check2 = [[0, 0, 0, 0], [0, 0, 0,0]] - Creates two separate lists [0,0,0,0]. Row 1 and Row 2 refers to different copies of [0,0,0,0]

So changing the elements of one row will not reflect any changes in the other row.

Please read this SO answer to fully understand the concepts. https://stackoverflow.com/a/37830340

Upvotes: 2

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