[Tutor] how to make a reference to part of list?

[Xie Chao]

3Q very much!
   What I really want to get is a c-style array, a pointer to the beginning of part of a list (or whatever alike). for example, I want to mutate part of a list, of course, I can achieve this by passing the pair list and (begin, end) as parameters. but in some case, this manner is quite tedious and in efficient (I think so, at least in C-style language it is so)!
    I don't wanna get the '+' operator (which is powerful tool in scripting language), so I ignore this problem. thank you all the same!
    maybe this prolem looks silly, since I've just begun to study scripting language. 


----- Original Message ----
From: Kent Johnson <kent37 at tds.net>
Cc: tutor at python.org
Sent: Monday, October 2, 2006 7:57:31 PM
Subject: Re: [Tutor] how to make a reference to part of list?


Xie Chao wrote:
>       Namely, if  list1 = {1, 2, 3}, and I wanna make a reference, say 
> "ref",  to list1's sublist (which is also of type list), say {2, 3}, so 
> that when I make change to ref[0], then list1[1] will be changed!
>       But how can I make such a reference, 3x!  

There is nothing built-in to Python that will let you do this. I don't 
think it would be too hard to write a class that does what you want for 
basic indexing operations. I would start with a copy of UserList (in 
module UserList). If you go beyond simple indexing and slicing I think 
you will have trouble. Say you have a class RefList that does what you 
want, and suppose you have
a = [1, 2, 3]
b = RefList(a, 1, 2)
b is [2, 3]
c = RefList(a, 0, 1)
c is [1, 2]

Now what is the result of d=b+c? Is it a RefList with two references to 
a, that looks like [1, 2, 2, 3]? What is the value of d after d[1] = 0? 
Is it [1, 0, 0, 3]?

Can you say more about why you want to do this? Maybe there is another 
solution.

By the way list literals in Python are written with square brackets [] 
not curly braces {}.

Kent


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