[Tutor] Initializing a list as an array

[Kalle Svensson]

[Scott]
> On Friday 07 December 2001 07:24 pm, you wrote:
> >
> > >Suppose I want to initialize a list that I want to use like an
> > >array, setting the initial value of each element to zero.
> > >
> >    >>> mylist = [0]*10
> >    >>> mylist
> >    [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
> 
> I thought it would be something like that.  I played around with several 
> variations, but couldn't get it to work.  Thanks Kirby.

Just a little warning here, this doesn't extend to multidimensional lists:

>>> mymatrix = [[0]*3]*3
>>> mymatrix
[[0, 0, 0], [0, 0, 0], [0, 0, 0]]

Seems nice enough, eh?  But:

>>> mymatrix[0][0] = 1
>>> mymatrix
[[1, 0, 0], [1, 0, 0], [1, 0, 0]]

The reason is that list are mutable objects (i.e. their contents can be
changed during the execution of the program).  Integers (such as 0) are not.
mylist = [0] * 3
creates a list containing three references to the object 0.
myarray = [[0] * 3] * 3
creates a list containing three references to a list object containing three
references to the object 0.

Weird?  Not helping?  Sorry. <wink>
In fact, this (references and mutable/immutable objects) is one of the things
I like very much about Python, but it took me a while to understand it.

Finally, code that does the thing you might expect from the example above:

>>> mymatrix = [[0] * 3 for x in xrange(3)]
>>> mymatrix[0][0] = 1
>>> mymatrix 
[[1, 0, 0], [0, 0, 0], [0, 0, 0]]

Oh, and I'm sure this is in the FAQ.  Hope you don't mind my posting anyway.

Peace,
  Kalle
-- 
Kalle Svensson (kalle@gnupung.net) - Laziness, impatience, hubris: Pick two!
English: http://www.gnupung.net/  Svenska: http://www.lysator.liu.se/~kalle/
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