Is it possible to have duplicate in QSet/Hash ?

AntGP

Hello,

I just wanted to ask a simple question to confirm what I'm trying to understand.

From what I have understood so far, Qset/Hash documentation doesn't say anything about the potentiality of a duplicate, but the way they are made, if a data is inserted in a Qset on which the data already exists, it will not create a second entry on the QSet ? Otherwise, with the arbitrary order of data storage, it would be impossible to differentiate them.

So my question based on this understanding is : If I don't want duplicates (compared to the case of a QList), is it always true that if I have two exact entries I have no duplicates ?

Exemple :


QSet<QString> set;

set.insert("one");

set.insert("three");

set.insert("one");

In this case, inspried by the one of the documentation, I will only have "one" and "three" stored ?

And will be the case for every type of data, even in the case of a Qset<QListe<QString>> ? (the one I actually want to use)

I did not found where this propriety even if logic is stated, if it exist at all.

Pl45m4

@AntGP said in Is it possible to have duplicate in QSet/Harsh ?:

Qset/Harsh documentation doesn't say anything about the potentiality of a duplicate, but the way they are made, if a data is inserted in a Qset on which the data already exists, it will not create a second entry on the QSet ?

At least for QHash it does...

QHash is unordered, so an iterator's sequence cannot be assumed to be predictable. If ordering by key is required, use a QMap.

Normally, a QHash allows only one value per key. If you call insert() with a key that already exists in the QHash, the previous value is erased. For example:
hash.insert("plenty", 100);

hash.insert("plenty", 2000);

// hash.value("plenty") == 2000

(https://doc.qt.io/qt-6/qhash.html)

That also includes assigning the same value again... which means the QHash pair is updated (to the same value but that does not matter here)... So no multiple values for the same hashed key.

For this there is QMultiHash, which allows multiple values per key.

Edit:

Because QSet has an internal QHash table but no key, I think the same applies.

AntGP

Thx ! it answered my question perfectly !

Pl45m4

@AntGP said in Is it possible to have duplicate in QSet/Harsh ?:

Thx ! it answered my question perfectly !

Great!

@AntGP said in Is it possible to have duplicate in QSet/Harsh ?:

even in the case of a Qset<QListe<QString>> ? (the one I actually want to use)

For convenience use QSet<QStringList> instead, which is exactly a QList of QStrings plus some extra features

AntGP

It would have been a good suggestion , but.... i'm not the one who made the Qlist<Qstring>... i'm trying to look at something 12 years old, made over 10 years by at least 3 differents persons whit no doc

Pl45m4

@AntGP said in Is it possible to have duplicate in QSet/Harsh ?:

i'm not the one who made the Qlist<Qstring>

That should not matter?!
But you are trying to put that QList<QString> construct in a container of your choice (QHash / QMap / QSet) right?
So you can still make it QSet<QStringList> and add the old data to it, in fact QStringList IS a QList<QString>, with its own API on top.

https://doc.qt.io/qt-6/qstringlist.html#details

Of course you don't have to... but sometimes things like

QStringList() << "Foo" << "Bar" << "42" << "Mooh";

as well as joins and filters, come quite handy :)

AntGP

To be honnest, looking more deeply in Qhash than i'v done before, it appered that it was the perfect tool for a bit later.
Once my thing is working and I have fixed my own mess, I'll take a look at improving what mess have been made before me.