Quiz Time! Prepending items to QVector

VRonin

How many elements?

The benchmark tested 10, 50, 100, 500, 1000, 5000 but turns out the answer doesn't change with size

can I use reserve?

Yes, in both cases I first called QVector::reserve

Are we talking 32 or 64 bits?

64

I would expect using a reverse iterator for reading and append for writing would beat both strategies mentioned in the initial post.

Yes, this is obvious but in my use case I can't read with reverse iterator

or does this also need to have the initial sequence reversed prior to the append + reverse operation?

To make it more explicit: If I have the sequence 1, 3, 2, 5 (that I can't reverse iterate) I want the vector to store those as 5, 2, 3, 1

VRonin

And the answer is:
In Qt5 append+std::reverse is vastly superior, In Qt6 prepend is the much faster one.

fcarney

What about just starting at the end of the data first and only appending?

VRonin

@fcarney said in Quiz Time! Prepending items to QVector:

What about just starting at the end of the data first and only appending?

I guess you mean in Qt6. In that case append is slightly better but the difference is really tiny

SimonSchroeder

Is there any article about these specific changes in Qt 6 that I can read? What exactly has changed between Qt 5 and 6?

Christian Ehrlicher

@SimonSchroeder said in Quiz Time! Prepending items to QVector:

What exactly has changed between Qt 5 and 6?

QVector is no longer in Qt6 and an alias for QList. I would guess QList in Qt5 will also show the same fast results for prepend since QList has an optimization for this usecase since ages.

VRonin

From https://wiki.qt.io/New_Features_in_Qt_6.0

QVector and QList are unified. QList is updated and should be used by default when array-like behaviour is needed
QList, QString and QByteArray now have optimized complexity of insertion at the beginning (a.k.a. prepend)

kshegunov

@VRonin I'm late to the party, as usual ...
... and also as usual I probably sound like a broken record, but what your task sounds like is actually a stack, not a vector ... ;)

VRonin

@kshegunov said in Quiz Time! Prepending items to QVector:

but what your task sounds like is actually a stack, not a vector

Indeed, this was an iper-semplification of my problem to solve the academic dilemma.
I agree that a stack or, as @jeremy_k already pointed out, using reverse iterators would achieve the same

Dixon154

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