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Re: Creating a list with holes

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  Re: Creating a list with holes Chris Angelico <rosuav@gmail.com> - 2014-01-04 02:37 +1100
    Re: Creating a list with holes Roy Smith <roy@panix.com> - 2014-01-03 10:51 -0500
      Re: Creating a list with holes Chris Angelico <rosuav@gmail.com> - 2014-01-04 02:57 +1100

#63062 — Re: Creating a list with holes

On Sat, Jan 4, 2014 at 2:19 AM, Larry Martell <larry.martell@gmail.com> wrote:
> I think I know the answer is no, but is there any package that allows
> creating a list with holes in it? E.g. I'd want to do something like:
>
> x[10] = 12
> x[20] = 30
>
> I'm thinking of something like defaultdict but for lists (I know
> that's very different, but ... )

Depending on what exactly you need, it's probably worth just using a
dict. In what ways do you need it to function as a list? You can
always iterate over sorted(some_dict.keys()) if you need to run
through them in order.

Alternatively, if you expect to fill in most of the elements, it's
possible you'd be happier working with a subclass of list that
auto-expands by filling in the spare space with a singleton meaning
"no element here". The code probably exists somewhere, but if not, it
wouldn't be hard to write. Then it'd be a list, but you can start with
it empty and assign as you describe above.

What's the use case? I expect that one or the other of those options
would cover most cases, but maybe yours is different.

ChrisA

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#63068

In article <mailman.4853.1388763434.18130.python-list@python.org>,
 Chris Angelico <rosuav@gmail.com> wrote:

> Alternatively, if you expect to fill in most of the elements, it's
> possible you'd be happier working with a subclass of list that
> auto-expands by filling in the spare space with a singleton meaning
> "no element here".

And, if you know ahead of time the maximum number of elements you will 
ever have:

x = [None] * max_count

will preallocate them all with a minimum of fuss.

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#63070

On Sat, Jan 4, 2014 at 2:51 AM, Roy Smith <roy@panix.com> wrote:
> In article <mailman.4853.1388763434.18130.python-list@python.org>,
>  Chris Angelico <rosuav@gmail.com> wrote:
>
>> Alternatively, if you expect to fill in most of the elements, it's
>> possible you'd be happier working with a subclass of list that
>> auto-expands by filling in the spare space with a singleton meaning
>> "no element here".
>
> And, if you know ahead of time the maximum number of elements you will
> ever have:
>
> x = [None] * max_count
>
> will preallocate them all with a minimum of fuss.

Yes, as I use in the trivial example in the subsequent post. But as a
general solution this is usually insufficient. (Whether or not None is
valid as a sentinel is, of course, quite orthogonal to the discussion.
I avoided assuming that it was, the OP's now shown that it does seem
to be.)

ChrisA

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