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From: Malcolm McLean <malcolm.arthur.mclean@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: Baby X is bor nagain
Date: Wed, 19 Jun 2024 17:52:11 +0100
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On 19/06/2024 11:42, David Brown wrote:
> On 19/06/2024 11:25, Malcolm McLean wrote:
>> On 18/06/2024 23:49, Keith Thompson wrote:
>>> Malcolm McLean <malcolm.arthur.mclean@gmail.com> writes:
>>> [...]
>>>> 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.
>>>
>>> <OT>
>>> https://docs.python.org/3/library/functions.html#sorted
>>> https://docs.python.org/3/library/stdtypes.html#list.sort
>>> </OT>
>>>
>> key specifies a function of one argument that is used to extract a 
>> comparison key from each element in iterable (for example, 
>> key=str.lower). The default value is None (compare the elements 
>> directly).
>>
>> You see the problem. I can sort on any field. I can sort alphaetically 
>> upwards and downwards. But I don't want to do that. I want to use a 
>> non-alphabetical comaprison function on two fields, and I need to 
>> specify that myself, because it's impossible that it is available 
>> anywhere. And that is to sort alphalbetically, except where the 
>> strings match except for an emedded "_L_" or "_D_" where the string 
>> wth the embedded "L" shoud be treated as closer to A than the string 
>> with the emebdded "_D_".
>>
> 
> def LD_key(n) :
>      if "_L" in n : return (0, n)
>      if "_D_" in n : return (1, n)
>      return (2, n)
> 
> Now you have a key function that will put all names containing "_L_" 
> first, then all names containing "_D_", then everything else, with 
> alphabetic sorting within those groups.
> 

Yes, but that's not quite what we want. A typical input would go.

quill_icon_D_.png
quill_icon_D_@2x.png
quill_icon_L_.png
quill_icon_L_@2x.png
aardvark.png
zebra.png

and the ouput we want is

aardvark.png
quill_icon_L_.png
quill_icon_D_.png
quill_icon_L_@2x.png
quill_icon_D_@2x.png
zebra.png

> There is no problem here - you just have to think about things in a 
> different way.
> 
> (I don't know why Python 3 dropped the comparison function support from 
> sort()/sorted().  It might be that a key function is more efficient, 
> since you call it once for each item rather than once for each comparison.)
> 
>> And I'm sure there is some way to achiev e this. But in C, it s 
>> achieved simply by declaring that qsort takes a function pointer to 
>> user-supplied code.
>>
> 
> Yes, there is some way to achieve this all in Python.  And like pretty 
> much every other question that is commonly asked, google will tell you 
> the answer.  Sometimes things seem hard - then you do a little research, 
> learn a bit, and then its easy.
> 
> 
I struugled with it for a while and, as you say, had recourse to the 
web. It was the very first Python program I tried to write, and it 
semmed unacceptably difficult to carry out a simple customised sort. I 
got alphabetical sorting going quite easily, but you don't need 
scripting for that.

It wasn't what I was ment to be spending my time on anyway. So I had to 
drop the idea of automating the collation of resources with a Python 
script.
-- 
Check out my hobby project.
http://malcolmmclean.github.io/babyxrc