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I want to have square matrix of integers with dimensions 3x3. I don't want to use numpy. There are several ways how to initialize such structure. For example:

Direct definition (explicitly written out the whole structure):

a = [ [0,0,0], [0,0,0], [0,0,0] ]

and

b = [[0]*3]*3

When I type 'a' or 'b' in python3 interpreter I got same results (str function for both objects prints out the same):

>>> a
[[0, 0, 0], [0, 0, 0], [0, 0, 0]]
>>> b
[[0, 0, 0], [0, 0, 0], [0, 0, 0]]

however, when I try to set for example number on position row, column = 0, 1 I get different results:

>>> a[0][1] = 1
>>> b[0][1] = 1
>>> a
[[0, 1, 0], [0, 0, 0], [0, 0, 0]]
>>> b
[[0, 1, 0], [0, 1, 0], [0, 1, 0]]

How so? What's special about [[0]*3]*3?

Vladimir Fekete
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1 Answers1

1

The second one creates three rows that are addressing the same memory. This way any change in one of them reflects to them all. The first one, on the other side, creates three separate lists

Enrico Agrusti
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