How to modify qmap without iteration

[sanjarbek]

How to modify QMap without iteration,

In my code I verify qmap if it has a key.
If no then I insert a new key and value. If yes then I want to increment a value.

How to do this?

Sorry for my English.

[spud]

if(!map.contains(key))
    map.insert(key, defaultValue);
else
    map[key]++;

if(!map.contains(key))
    map.insert(key, defaultValue);
else
    map[key]++;

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[sanjarbek]

if(!map.contains(key))
    map.insert(key, defaultValue);
else
    map[key]++;

if(!map.contains(key))
    map.insert(key, defaultValue);
else
    map[key]++;

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yes, it works, but I tried this

QMap<QString, int > *temp = new QMap<QString,  int >;
temp->insert("first", 1);
temp["first"] = 2;

QMap<QString, int > *temp = new QMap<QString,  int >;
temp->insert("first", 1);
temp["first"] = 2;

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and get this error

C:/QtProjects/NaiveBayesian/attribute.cpp:42: error: invalid types 'QMap<QString, int>*[const char [6]]' for array subscript

[spud]

This is an ugly C++ pitfall. Since temp is a pointer, you would have to write:

(*temp)["first"] = 2;

(*temp)["first"] = 2;

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but an altogether better approach would be:

QMap<QString, int > temp;
temp.insert("first", 1);
temp["first"] = 2;

QMap<QString, int > temp;
temp.insert("first", 1);
temp["first"] = 2;

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See implicit sharing.

[sanjarbek]

Thank you, it is working, but it seems some ugly.

I need to create QMap variable dynamically.

[spud]

I need to create QMap variable dynamically.

Dynamically, sure. On the heap, probably not.
Have you read about implicit sharing?

[sanjarbek]

Dynamically, sure. On the heap, probably not.
Have you read about implicit sharing?

Yes, I have read it before.
In my program I have a class which have 3 members: bool, QMap, int.
In the case of bool variable I need either QMap or int.
This is why I want to create variables dynamically.

[sanjarbek]

I changed my code.

QMultiMap<QString, QMap<QString, int> > mTable;
    if (mTable.value(instancevalue).contains(classname))
        mTable[instancevalue][classname]++;
    else
        mTable[instancevalue].insert(classname, 1);

QMultiMap<QString, QMap<QString, int> > mTable;
    if (mTable.value(instancevalue).contains(classname))
        mTable[instancevalue][classname]++;
    else
        mTable[instancevalue].insert(classname, 1);

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get this error message

C:/QtProjects/NaiveBayesian/../../Qt/2010.02.1/qt/include/QtCore/../../src/corelib/tools/qmap.h:1013: error: 'T& QMultiMap<Key, T>::operator[](const Key&) [with Key = QString, T = QMap<QString, int>]' is private

C:/QtProjects/NaiveBayesian/../../Qt/2010.02.1/qt/include/QtCore/../../src/corelib/tools/qmap.h:1013: error: 'T& QMultiMap<Key, T>::operator[](const Key&) [with Key = QString, T = QMap<QString, int>]' is private

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[spud]

See the documentation of QMultiMap:

Unlike QMap, QMultiMap provides no operator[]. Use value() or replace() if you want to access the most recently inserted item with a certain key

[sanjarbek]

I understood. Thank you for your help. Until now I used dotnet and now I pass to qt. My C++ background is good, just I get accustomed dotnet style model.