Why changing an element of a matrix is changing both rows?

Question

I have the following code.

x = [['']*2]*2 # [['', ''], ['', '']]
x[0][0] = 'a'

This produces the following. But why? When I actually just changed the first element of the first list.

[['a', ''], ['a', '']]

Or is [['']*cols]*rows not the right way to create a matrix in Python?

the inner * is ok since replicates an immutable string. But the outer * replicates a reference to a mutable list. When you change one, you change both. It's a repeat without copy. — hpaulj, Jan 18 '20 at 21:57

Nader Alexan · Accepted Answer · 2020-01-18T18:59:19.913

1

Because python is using the same list twice and just referencing it.

You can use the function copy() for creating copies or use numpy for creating empty matrix (check https://stackoverflow.com/a/13347614/1223945)

edited Jan 18 '20 at 18:59

answered Jan 18 '20 at 18:56

Nader Alexan

Ah OK. I think I can use a list comprehension or a map function then. – MetallicPriest Jan 18 '20 at 18:57
I ended up using list(map(lambda row: [None] * cols, [None] * rows)) – MetallicPriest Jan 18 '20 at 19:02
Yes you can. You can also use Numpy – Nader Alexan Jan 18 '20 at 19:02
@MetallicPriest if my answer helped with your question, please mark it as the correct answer. – Nader Alexan Jan 22 '20 at 13:02

1 Answers1