[fpc-devel] Sorting tests

[J. Gareth Moreton]

Quicksort is not inherently stable because of what happens if a value is 
equal to the pivot - more specifically, which of the identical values is 
selected as the pivot - for example, if you try to sort a list of 
identical values, they will inevitably change order because each stage 
will move all of the values to one side of the pivot (and also cause 
quicksort to degenerate into O(n²)).  If you want a stable sort, you 
need to use things like merge sort instead.

Kit

On 29/11/2022 13:25, Sven Barth via fpc-devel wrote:
> Ondrej Pokorny via fpc-devel <fpc-devel at lists.freepascal.org> schrieb 
> am Di., 29. Nov. 2022, 11:39:
>
>     Am 29.11.2022 um 11:08 schrieb Sven Barth via fpc-devel:
>>     J. Gareth Moreton via fpc-devel <fpc-devel at lists.freepascal.org>
>>     schrieb am Di., 29. Nov. 2022, 10:09:
>>
>>         Surely that's a bug in the comparison functions that should
>>         be fixed and
>>         not something that can be blamed on introsort.  If a
>>         comparison function
>>         is faulty, then pretty nuch any sorting algorithm can be
>>         considered to
>>         have unpredictable behaviour.
>>
>>
>>     This *can* be blamed on IntroSort, because Stability (order of
>>     equal elements is kept) is an attribute of sorting algorithms and
>>     IntroSort is *not* considered stable while QuickSort *can* be
>>     stable depending on the implementation and ours *is*.
>
>     If for two elements [a, b] the comparison function returns
>     a<b=true and b<a=true
>
>     then the problem is not in stability or any other feature of the
>     sorting algorithm but in the comparison function indeed. Or am I
>     missing something?
>
>
> For such a comparison function the issue is indeed in the comparison 
> function, but Nikolay also mentioned "a<b=false and b<a=false" which 
> is the case for equal elements.
>
> Regards,
> Sven
>
>
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