QHash, how to update value in map?

SPlatten

I have an instance of QHasp:

typedef QHash<QString, quint64> tProgressMap;

In a class:

tProgressMap mhmTrainees;

In the method that updates progress:

tProgressMap::iterator itFound(mhmpTrainees.find(crstrTrainee));
if ( itFound == mhmpTrainees.end() ) {
  itFound = mhmTrainees.insert(crstrTrainee, uint64Value);
} else {
  itFound.value() = uint64Value;
}

Whilst:

  itFound.value() = uint64Value;

Updates the value because value() returns a reference, it does not update the same entry in the mhmpTrainees.

Is there a way to do this or do I have to remove and reinsert?

ollarch

Hi,

Use [] operator.

SPlatten

@ollarch , can you show a demonstration please?

Tried:

mhmTrainees[itFound] = uint64Value;

Compile errors:

C2679: binary '[': no operator found which takes a right-hand of type 'QHash<QString, quint64>::iterator'

Edit...

mhmTrainees[itFound.key()] = uint64Value;

This works!

JonB

@SPlatten
I'm not a wiz with iterators, but you should be able to do it from the right iterator (without having to re-look-up the key). See https://doc.qt.io/qt-5/qhash-iterator.html#details

Let's see a few examples of things we can do with a QHash::iterator that we cannot do with a QHash::const_iterator. Here's an example that increments every value stored in the QHash by 2:

QHash<QString, int>::iterator i;
for (i = hash.begin(); i != hash.end(); ++i)
    i.value() += 2;

SPlatten

@JonB , the problem is that the iterator itself is not a pointer or a reference so there is no link to the original data source it references, I've resolved this now.

KroMignon

@SPlatten said in QHash, how to update value in map?:

the problem is that the iterator itself is not a pointer or a reference so there is no link to the original data source it references, I've resolved this now.

the iterator holds a reference to the item (key and value)!

The code given by @JonB is the right way to do (cf. https://doc.qt.io/qt-5/qhash-iterator.html#details)
mhmTrainees[itFound.key()] = uint64Value; should be itFound.value() = uint64Value;

SPlatten

@KroMignon , that was exactly what my original code looked like and it doesn't work, I can see whilst the iterator is modified the original value in the QHash isn't modified and I'm not surprised because .begin() doesn't return a reference it just returns an iterator so there is no link.

jsulm

@SPlatten said in QHash, how to update value in map?:

begin() doesn't return a reference it just returns an iterator so there is no link

As already said by @KroMignon the iterator holds the reference...
It is also shown in https://doc.qt.io/qt-5/qhash-iterator.html

mpergand

Sorry @SPlatten but it works as @JonB said

    typedef QHash<QString,int> H;
    H h;
    h["A"]=1;
    h["B"]=2;
    
    for( auto& i: h)
        {
        i++;
        }
    
    qDebug()<<h;
    
    QHash<QString, int>::iterator i;
    for (i = h.begin(); i != h.end(); ++i)
        {
        i.value()++;
        }
    
    qDebug()<<h;

Logs:
QHash(("B", 3)("A", 2))
QHash(("B", 4)("A", 3))

SPlatten

@mpergand , don't apologise, this isn't what I am seeing in Qt Creator / Debugger.

jsulm

@SPlatten Then you are doing something wrong because I just tried the code from @mpergand and it works as expected...

SPlatten

@jsulm , ok, just tried it again and of course it is working as you say, not sure how or how I got it so wrong...

KroMignon

@SPlatten said in QHash, how to update value in map?:

don't apologise, this isn't what I am seeing in Qt Creator / Debugger.

I am very surprised!
I always do it like this (it.value() = newValue) and it is the way it is documented.
Can you show how you have set the iterator?
It is an iterator or const_iterator?