[Python-ideas] List Revolution

[Eric Snow]

On Sat, Sep 10, 2011 at 7:51 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On Sun, Sep 11, 2011 at 10:37 AM, Steven D'Aprano <steve at pearwood.info> wrote:
>> 0-based indexes are useful for some tasks, and less useful for other tasks.
>> In my experience, I find that 0-based indexing is more useful most of the
>> time: it leads to fewer off-by-one errors.
>>
>> 1-based indexes are particularly well-suited for programming languages using
>> a natural language metaphor, usually aimed at non-programmers. Examples
>> include Xion, Applescript, and Inform-7.
>
> Indeed, the concepts of half-open ranges and 0-based indexing go hand
> in hand (as described in the EWD article), and it ties directly in to
> the notion of *index arithmetic*.
>
> A case that illustrates this nicely is that of partitioning a
> sequence. Suppose we want the first 5 items in one subsequence and the
> rest in another. This is easy to write, and requires no adjustments to
> the numbers:
>
>  head = seq[:5]
>  assert len(head) == 5
>  tail = seq[5:]
>  assert len(tail) == len(seq) - 5
>
> Zero based indexing (in conjunction with half-open ranges) makes the
> arithmetic work out nicely, and, in practice, that turns out to be
> important when it comes to writing correct programs.
>
> However, it comes at the cost of breaking the intuitive mapping to the
> counting numbers: the first item is at offset 0, the second is at
> offset 1, etc. This is a definite downside, but the collective
> judgment of many language designers is that the reduction in
> off-by-one errors when manipulating indices is worth the additional
> difficulty in learning the language for programming novices.

This discussion has reminded me of a post from Tim Peters I stumbled
on a few weeks ago:

    http://mail.python.org/pipermail/python-list/2000-October/637704.html

"The trick is that indices in Python point *between* array elements"...

Terry Reedy also explained this pretty well, I thought:

    http://mail.python.org/pipermail/python-list/2005-September/927859.html

-eric



* Tim also had an earlier, but similar post that I found when looking
for the remembered one:

    http://mail.python.org/pipermail/python-list/1999-August/622024.html

>
> Cheers,
> Nick.
>
> --
> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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