Re: sort - C, python, C++, glibc Was: Baby X is bor nagain

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