Changing the same element in all lists in a 2D-array [duplicate]

Question

When doing something like for example:

arr = [[0]*3]*3
arr[1][0] = 4

It will alter the first element in all three lists. Is this because we are only making a copy of the first list and the elements share the same pointer in memory?

That is, the first list is for example:

[A,A+C,A+2C] where C is the Offset and A is the address to the first value. And the construction above creates a list: [[A,A+C,A+2C], [A,A+C,A+2C], [A,A+C,A+2C]] Ergo, if we change any element in any position of one of the lists, we alter the corresponding the the other lists?

stackoverflow.com/questions/9954183/…

chrslg
– chrslg

2022-10-29 09:39:07 +00:00
Commented Oct 29, 2022 at 9:39 — chrslg
– chrslg, Commented Oct 29, 2022 at 9:39

Yuri Feldman · Accepted Answer · 2022-10-29 09:38:23Z

1

Exactly. The repeat command *3 Makes a shallow copy, in all instances. Something to watch out for. I personally have been bitten by this in initialization, e.g.

a, b = []*2

Makes a and b refer to the same object.

answered Oct 29, 2022 at 9:38

Yuri Feldman

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