Question

Possible Duplicate:
Python list problem

I try to initialise a matrix in python.
First I did this:

>>> M=[[0]*4]*4

But here is my probleme, every line is changing when I change the first one:

>>> M
[[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]
>>> M[1][1]=1
>>> M
[[0, 1, 0, 0], [0, 1, 0, 0], [0, 1, 0, 0], [0, 1, 0, 0]]

So I did it this way:

>>> M= [ [ 0 for i in range(4) ] for j in range(4) ]

And ut works fine:

>>> M
[[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]
>>> M[1][1]=1
>>> M
[[0, 0, 0, 0], [0, 1, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]

My question is:

What these two expressions really mean ? ans why the first one is behaving this way ?

Thanks in advance for you help.

Answer 1

Say a is some python object. Then [a] * 4 is equivalent to [a, a, a, a]. What this means depends on whether a is mutable or not. Numbers, strings and tuples are not, so if a is one of these kinds of objects (0 in your example), then you get 4 independently changeable copies. Lists, dictionaries and sets are mutable, and in this case you just get 4 references to the same object, in your case the list [0] * 4. Exploiting this knowledge, you’ll see that you can do this:

M = [[0] * 4 for i in range(4)]

and get what you want.