Performance: sets vs dicts.

Arnaud Delobelle arnodel at googlemail.com
Wed Sep 1 22:46:12 CEST 2010


Terry Reedy <tjreedy at udel.edu> writes:

> On 9/1/2010 11:40 AM, Aahz wrote:
>> I think that any implementation
>> that doesn't have O(1) for list element access is fundamentally broken,
>
> Whereas I think that that claim is fundamentally broken in multiple ways.
>
>> and we should probably document that somewhere.
>
> I agree that *current* algorithmic behavior of parts of CPython on
> typical *current* hardware should be documented not just 'somewhere'
> (which I understand it is, in the Wiki) but in a CPython doc included
> in the doc set distributed with each release.
>
> Perhaps someone or some group could write a HowTo on Programming with
> CPython's Builtin Classes that would describe both the implementation
> and performance and also the implications for coding style. In
> particular, it could compare CPython's array lists and tuples to
> singly linked lists (which are easily created in Python also).
>
> But such a document, after stating that array access may be thought of
> as constant time on current hardware to a useful first approximation,
> should also state that repeated seqeuntial accessess may be *much*
> faster than repeated random accessess. People in the high-performance
> computing community are quite aware of this difference between
> simplified lies and messy truth. Because of this, array algorithms are
> (should be) written differently in Fortran and C because Fortran
> stores arrays by columns and C by rows and because it is usually much
> faster to access the next item than one far away.

I don't understand what you're trying to say.  Aahz didn't claim that
random list element access was constant time, he said it was O(1) (and
that it should be part of the Python spec that it is).

-- 
Arnaud



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