[Interest] How to make the QString "10" behind "9" in QMap's key order

[Carel Combrink]

Hi Mark,

How would one use QCollatorSortKey for this type of application?

Unfortunately I have to say that the Qt documentation on this class is not
as descriptive compared to other classes.

Regards,
Carel

On Mon, Jan 8, 2018 at 11:50 AM, Mark Gaiser <markg85 at gmail.com> wrote:

> On Mon, Jan 8, 2018 at 4:18 AM, jack ma <assangema at gmail.com> wrote:
>
>> Thanks all of you, seems that std::sort + QCollator is a good way
>>
>> 2018-01-07 18:47 GMT+08:00 Konstantin Tokarev <annulen at yandex.ru>:
>>
>>>
>>>
>>> > Hi,
>>> >
>>> > you need a "lessThan"-function, tailored for your needs. An
>>> > example from me is this:
>>> >
>>> > bool Person::lessThanFamilyFirst(QSharedPointer<Person>
>>> > p1, QSharedPointer<Person>
>>> > p2)
>>> >
>>> > {
>>> >
>>> > return (QString::localeAwareCompare(p1->familyName(),p2->familyName
>>> ())<0)
>>> >
>>> > || (( p1->familyName() == p2->familyName())
>>> >
>>> > && (QString::localeAwareCompare(p1->firstName(),p2->firstName())<0));
>>> >
>>> > }
>>> >
>>> > Now you can pass this as a functor to a sort method like this:
>>> >
>>> > std::stable_sort(pupils.begin(), pupils.end(),
>>> Person::lessThanFamilyFirst);
>>>
>>> FWIW, if this is not a list displayed in UI, or other place where
>>> preserving order of
>>> equal elements matters, you should consider std::sort
>>>
>>> >
>>> > Please notice, that there are no () when calling the lessThan functor.
>>>
>>> Original question was dealing with QMap. While sorted vector-like
>>> container can replace
>>> map (there is even convenience wrapper flat_map in Boost, which allows
>>> to work with
>>> sorted array just like it was a "true" map), it doesn't always fit, e.g.
>>> inserts in such
>>> "flat map" are very expensive.
>>>
>>> So, you can just replace QMap with std::map and pass your comparator,
>>> e.g.
>>>
>>> auto comparator = [](const QString &a, const QString &b) -> bool {
>>>     // Your comparison function here
>>> };
>>> std::map<QString, QGraphicsItem *, decltype(comparator)>
>>> myMap(comparator);
>>>
>>> See [1] for more examples.
>>>
>>> Yet another approach is to use your own type as a key of QMap instead of
>>> QString,
>>> and create desired operator< for it. This has a downside that your
>>> access to internal
>>> QString will be more complicated, and you'll have to invent different
>>> key type each time
>>> you want to change comparator.
>>>
>>> And, last but not least: you probably want to know *how* to write
>>> comparator for your
>>> particular sorting order. You can do this with QCollator, see [2].
>>>
>>> [1] http://en.cppreference.com/w/cpp/container/map/map
>>> [2] https://forum.qt.io/topic/68910/natural-sort-using-qcollator/2
>>>
>>> >
>>> > HTH
>>> > Sebastian
>>> >
>>> > Am 07.01.2018 um 09:01 schrieb jack ma:
>>> >
>>> >> Hi,
>>> >>
>>> >> there is a QMap<QString,QGraphicsItem *> type, then
>>> >> insert values with：
>>> >>
>>> >> type.insert("U1",nullptr),
>>> >>
>>> >> type.insert("U2",nullptr),
>>> >>
>>> >> …………
>>> >>
>>> >> type.insert("U9",nullptr),
>>> >>
>>> >> type.insert("U10",nullptr),
>>> >>
>>> >> I want get the values follow the order of U1, U2 ... U10，
>>> >> but the default order is U1, U10 ,……U9
>>> >>
>>> >> I know this is a common string sorting problem, but I do
>>> >> not know how to solve it well.
>>> >>
>>> >> Any suggestions are very grateful !
>>> >>
>>> >> Thanks.
>>> >>
>>> >> _______________________________________________
>>> >> Interest mailing list
>>> >> Interest at qt-project.org
>>> >> http://lists.qt-project.org/mailman/listinfo/interest
>>> >
>>> > --
>>> > http://www.classintouch.de - Tablet-Software für Lehrer
>>> --
>>> Regards,
>>> Konstantin
>>> _______________________________________________
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>>>
>>
>>
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>>
>>
> Hi,
>
> You might want to look into std::map[1] as well. It allows setting a
> compare function, QMap doesn't.
> Going that route is most likely more efficient as your inserts will be
> sorted, no need to do std::sort afterwards.
>
> You still would have to use QCollator. Note that you can just use
> QCollator in the compare method which will work just fine, but is not the
> performant way to go.
> The most performant way (i know of) is by using QCollatorSortKey[2].
>
>
> [1] http://en.cppreference.com/w/cpp/container/map/map
> [2] http://doc.qt.io/qt-5/qcollatorsortkey.html
>
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>
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