A simple question

[Tuvas]

I know the answer is probably really simple to this, and I feel bad to
even ask, but I can't find the answer anywhere... Let me show what
happened, then ask the question.
[color=blue][color=green][color=darkred]
>>> x=[[0]*2]*2
>>> x[/color][/color][/color]
[[0, 0], [0, 0]][color=blue][color=green][color=darkred]
>>> x[0][1]=1
>>> x[/color][/color][/color]
[[0, 1], [0, 1]][color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

The question now. Why is the output list [[0, 1], [0, 1]] and not [[0,
1], [0, 0]]? And how can I make it work right? I do know that x[0][1]
will always give the value of that specific coordinate, but never
before have I tried to manipulate rows like this, and I'm finding I
need to. Thanks!

[skip@pobox.com]

Re: A simple question

[color=blue][color=green][color=darkred]
>>> x=[[0]*2]*2[/color][/color][/color]

This replicates the references. It doesn't copy objects.

This short transcript demonstrates that concept:
[color=blue][color=green][color=darkred]
>>> x = [[0, 0], [0, 0]]
>>> map(id, x)[/color][/color][/color]
[16988720, 16988160][color=blue][color=green][color=darkred]
>>> y = [[0]*2]*2
>>> y[/color][/color][/color]
[[0, 0], [0, 0]][color=blue][color=green][color=darkred]
>>> map(id, y)[/color][/color][/color]
[16988520, 16988520]

The object x refers to is a list with references to two other lists. The
object y refers to is a list with two references to the same list.

Skip

[nnorwitz@gmail.com]

Re: A simple question

Skip answered why, but not how to make it work right:
[color=blue][color=green][color=darkred]
>>> x = [[0]*2 for x in range(2)]
>>> x[/color][/color][/color]
[[0, 0], [0, 0]][color=blue][color=green][color=darkred]
>>> x[0][1]=1
>>> x[/color][/color][/color]
[[0, 1], [0, 0]]

Cheers,
n

[Ben Cartwright]

Re: A simple question

Tuvas wrote:[color=blue]
> Why is the output list [[0, 1], [0, 1]] and not [[0,
> 1], [0, 0]]? And how can I make it work right?[/color]

http://www.python.org/doc/faq/programming.html#how-do-i-create-a-multidimensional-list

--Ben

[Tuvas]

Re: A simple question

Ahh, that make sense! Thanks a ton!