[Tutor] List indexing problem

[Mike Meisner]

Thanks Steve.

It's obvious now that you've pointed it out.

Do you happen to know if there is an efficient way to initialize a  list 
like this without explicitly writing out each element?

Mike
----- Original Message ----- 
From: "Steve Willoughby" <steve at alchemy.com>
To: "Mike Meisner" <mikem at blazenetme.net>
Cc: <tutor at python.org>
Sent: Friday, July 25, 2008 7:03 PM
Subject: Re: [Tutor] List indexing problem


> Mike Meisner wrote:
>> I need to do some statistical analysis by binning values into an array.
>>  Being new to Python, I tried to use a list of lists.  I've extracted 
>> just the minimum code that I'm having trouble with:
>
> What you need to remember is that Python works with *objects*, and 
> variables are simply *references* to those objects.  So if I say
> a = [1,2,3]
> b = a
>
> b and a both refer to the *SAME* list object (not two lists which happen 
> to have the same elements).
>
>>     temp = [[0, 0, 0],[0, 0, 0],[0, 0, 0]]
>
> temp now refers to a list object containing 3 lists, each containing 3 
> integers.
>
>>     # initialize to zero
>>     for i in range(20):
>>         IP.append(temp)
>
> Now IP contains 20 copies of references to the *same* list object.
> So you were modifying the underlying list object which happens to be 
> referenced many times in IP.
>
>