[Python-Dev] PEP 393 Summer of Code Project

[Steven D'Aprano]

Terry Reedy wrote:
> On 8/26/2011 8:23 PM, Antoine Pitrou wrote:
> 
>>> I would only agree as long as it wasn't too much worse
>>> than O(1). O(log n) might be all right, but O(n) would be
>>> unacceptable, I think.
>>
>> It also depends a lot on *actual* measured performance
> 
> Amen. Some regard O(n*n) sorts to be, by definition, 'worse' than 
> O(n*logn). I even read that in an otherwise good book by a university 
> professor. Fortunately for Python users, Tim Peters ignored that 
> 'wisdom', coded the best O(n*n) sort he could, and then *measured* to 
> find out what was better for what types and lengths of arrays. So not we 
> have a list.sort that sometimes beats the pure O(nlog) quicksort of C 
> libraries.

A nice story, but Quicksort's worst case is O(n*n) too.

http://en.wikipedia.org/wiki/Quicksort

timsort is O(n) in the best case (all items already in order).

You are right though about Tim Peters doing extensive measurements:

http://bugs.python.org/file4451/timsort.txt

If you haven't read the whole thing, do so. I am in awe -- not just 
because he came up with the algorithm, but because of the discipline Tim 
demonstrated in such detailed testing. A far cry from a couple of timeit 
runs on short-ish lists.



-- 
Steven