QMap or QHash doesn't return proper value

vijaychsk

Hi,
In my MainWindow.cc, I have this

struct Item {
     Item(QString path, QString type) : _path(path), _type(type) {}
     QString _path, _type;
}

QMap<QString, Item> _myDB;

I'm also populating _myDB in the same file using some method called push.

push(_search, MainWindow::Item(localPath, localType));

This push method internally calls insert of QMap.

In the ModelTable.cc, I'm trying to access the _myDB that was already populated. I just want to search and display the results in this file.

QVariant ModelTable::data(const QModelIndex& index, int role) const {
     Item o = _myDB.value(search); // here seems to be an issue.
     
    switch (o._type) { 
             case "EE" : return o._path; break;
             case "FE" : return o._path; break;
             default: break;
   }
}

However, I always get an empty string when I do QMap.value(key).

What am I doing wrong? Why does QMap doesn't return as expected? It is guaranteed that _myDB has some values and my search will always have a result. During the push operation, I can see what is being inserted. But when accessing the data(...), the results aren't as expected. I also tried using QMap<QString, QPair<QString, QString> instead of a struct, still the same issue.
QMap searching is somehow not working as expected.

eyllanesc

@vijaychsk what is the output of qDebug() << search << _myDB.keys();?

eyllanesc

@vijaychsk I did not reproduce the error:

#include <QMap>
#include <QDebug>

struct Item {
    Item(const QString & path="", const QString & type="") : _path(path), _type(type) {}
    QString _path, _type;
};

QDebug operator<<(QDebug debug, const Item &item)
{
    QDebugStateSaver saver(debug);
    debug.nospace() << '(' << item._path << ", " << item._type << ')';
    return debug;
}

int main()
{

    QMap<QString, Item> _myDB;
    _myDB["Foo"] = Item("foopath", "footype");
    _myDB["Bar"] = Item("barpath", "bartype");
    _myDB["Baz"] = Item("bazpath", "baztype");

    qDebug() << _myDB.value("Foo");
    qDebug() <<  _myDB.value("Bar");
    qDebug() <<  _myDB.value("Baz");
}

Output:

("foopath", "footype")
("barpath", "bartype")
("bazpath", "baztype")

vijaychsk

@eyllanesc
On search of QMap, we are getting a value of type Item.
Can you try assign the contents of the Item and try to print them?

QString output;
output = _myDB.value("foo")._path;
std::cout << output.toStdString() << std::endl;

In my code, I'm not using qdebug but doing assignment. Perhaps there might be some issue there.

jsulm

@vijaychsk said in QMap or QHash doesn't return proper value:

Item o = _myDB.value(search);

Are you sure there is such key in your _myDB?
What does _myDB.contains(search) return?
value() returns a default constructed value if there is no such key.

vijaychsk

@jsulm
_myDB.contains(search) is returning me empty. That is the problem.
But the search string exists. It is for sure.

This line is not working: Item o = _myDB.value(search);

ChrisW67

@vijaychsk said in QMap or QHash doesn't return proper value:

Item o = _myDB.value(search); // here seems to be an issue.

Where does "search" come from? It is not a parameter to the function, and it is not the same as "_search" that you used when you "push" an item onto the map. Is its value what you expect?

QMap does not have a push() function. Are you sure that what you implemented as push() is doing what you think?

eyllanesc

@vijaychsk what is the output of qDebug() << search << _myDB.keys();?

jsulm

@vijaychsk said in QMap or QHash doesn't return proper value:

_myDB.contains(search) is returning me empty.

What do you mean by "empty"? contains() returns a boolean. So, if it returns false then the map does not contain this key.
Print out all keys returned by https://doc.qt.io/qt-5/qmap.html#keys.
Do you by any chance have more than one QMap<QString, Item> in your code?

vijaychsk

Hi everyone,
Thank you for the responses. It was a naive mistake on my side.
I wrongly populated the _myDB.
When I did _myDB.keys() and _myDB.contains(), I got nothing. After I correctly entered the data, the results are as expected.