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From: Malcolm McLean <malcolm.arthur.mclean@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: sort - C, python, C++, glibc Was: Baby X is bor nagain
Date: Wed, 19 Jun 2024 18:42:42 +0100
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On 19/06/2024 11:53, David Brown wrote:
> On 19/06/2024 12:17, Michael S wrote:
>> On Wed, 19 Jun 2024 07:44:44 +0200
>> David Brown <david.brown@hesbynett.no> wrote:
>>
>>> On 18/06/2024 18:39, Malcolm McLean wrote:
>>>>>
>>>> My main experience of Python was that we had some resource files
>>>> which were icons, in matching light and dark themes. The light
>>>> theme had suffix _L followed by extension, and the dark themes had
>>>> _D. And they needed to be sorted alphabetically, except that _L
>>>> should be placed before _D.
>>>> And it didn't take long to get Python to sort the list
>>>> alphabetically, but there seemed no way in to the sort comparision
>>>> function itself. And I had to give up.
>>>
>>> Python "sort" is a bit like C "qsort" (desperately trying to relate
>>> this to the group topicality) in that you can define your own
>>> comparison function, and use that for "sort".  For simple comparison
>>> functions, people often use lambdas, for more complicated ones it's
>>> clearer to define a function with a name.
>>>
>>>
>>
>> Off topic:
>> Indeed, Python sort has option for specifying comparison function, but
>> I would not call it very similar to comparison function of C qsort
>> since in Python comparison is not applied directly to the record.
> 
> Yes, it seems that the comparison function support in sort() was in 
> Python 2 but was dropped for Python 3.
> 
This is exactly the sort of carry on which causes problems. You have a 
requirement for a custom sort. And, whilst I'm sure that the sort I 
ak=sked for is possible to achieve, its not exacty obvious how to 
achieve it from someone brought up on sort. So you trying looking it up 
on the web, and every answer is bases on Pyhtn 2 sort, which has been 
taken away.


>> Instead, it applied to the keys that are derived from record.
>> Besides, my impression is that in Python sorting by user-supplied
>> comparison function is less idiomatic than doing all heavy lifting in
>> user-supplied key function.
> 
> A key function can be applied once per item, while a comparison function 
> is called once per comparison - thus key functions will be (or at least 
> /can/ be) more efficient.
> 
>> For the case, presented by Malcolm, I'd certainly do it all in key(),
>> without custom cmp_to_key(). May be, it's a little less efficient, but
>> significantly easier to comprehend.
>>
>> Back to topic:
>> C qsort() sucks. They forgot to provide an option for 3rd parameter
>> (context) in comparison callback.
> 
>> Back to topic: why C standard committee still didn't add something like
>> gnu qsort_r() to the standard?
>>
> 
> That is a little more flexible, but it's still ugly!
> 
You seldom need a context pointer for sort functions. They must be pure 
functions, or at least they must return the same result for the same two 
copmarisions. And it's rare that it makes sense to give them extra 
paeameers.
-- 
Check out my hobby project.
http://malcolmmclean.github.io/babyxrc