QHash with key and struct problem

[MarkoSan]

Well, is it possible to get value of key if there is a value known from QHash?

Ok, that was another stupid question, it is all in docs ... there is a key() method ...

But, following method:

inline qint16 merchandiseId(const QString& strKey) const
        { return m_strNames.key(strKey).iLanguageId; };

inline qint16 merchandiseId(const QString& strKey) const
        { return m_strNames.key(strKey).iLanguageId; };

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produces following errors:

In file included from CMerchandizeElement.cpp:8:
CMerchandizeElement.h: In member function `qint16 CMerchandizeElement::merchandiseId(const QString&) const':
CMerchandizeElement.h:95: error: no matching function for call to `QHash<qint16, merchandiseLangStruct>::key(const QString&) const'
c:/Qt/4.4.3/include/QtCore/../../src/corelib/tools/qhash.h:652: note: candidates are: const Key QHash<K, V>::key(const T&) const [with Key = qint16, T = merchandiseLangStruct]
c:/Qt/4.4.3/include/QtCore/../../src/corelib/tools/qhash.h:658: note:                 const Key QHash<K, V>::key(const T&, const Key&) const [with Key = qint16, T = merchandiseLangStruct]
mingw32-make[1]: *** [debug/CMerchandizeElement.o] Error 1
mingw32-make: *** [debug] Error 2

In file included from CMerchandizeElement.cpp:8:
CMerchandizeElement.h: In member function `qint16 CMerchandizeElement::merchandiseId(const QString&) const':
CMerchandizeElement.h:95: error: no matching function for call to `QHash<qint16, merchandiseLangStruct>::key(const QString&) const'
c:/Qt/4.4.3/include/QtCore/../../src/corelib/tools/qhash.h:652: note: candidates are: const Key QHash<K, V>::key(const T&) const [with Key = qint16, T = merchandiseLangStruct]
c:/Qt/4.4.3/include/QtCore/../../src/corelib/tools/qhash.h:658: note:                 const Key QHash<K, V>::key(const T&, const Key&) const [with Key = qint16, T = merchandiseLangStruct]
mingw32-make[1]: *** [debug/CMerchandizeElement.o] Error 1
mingw32-make: *** [debug] Error 2

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Can someone help me please?

[yuriry]

Your hash is QHash<qint16, merchandiseLangStruct>, so the key() method needs to accept const merchandiseLangStruct&, and you are passing const QString&. You need to pass the struct itself.

[MarkoSan]

So what is the point then if I pass to this method whole struct, which already includes id?

[yuriry]

If you lookup by strMerchandiseName then they key should be QString.
If you lookup by iLanguageId, then the key should be qInt16.
If you lookup using both then you need 2 indexes. The index does not have to be QHash, it could be a simple sequential scan, but you need an index for every key.

[MarkoSan]

If you lookup by strMerchandiseName then they key should be QString.
If you lookup by iLanguageId, then the key should be qInt16.
If you lookup using both then you need 2 indexes. The index does not have to be QHash, it could be a simple sequential scan, but you need an index for every key.

Can you provide me please with some example?

[yuriry]

You already have QHash<qint16, merchandiseLangStruct>, use it to lookup by iLanguageId. Add another hash QHash<QString, merchandiseLangStruct> and use it to lookup by strMerchandiseName. Just be careful to update both hashes when you add/remove entries.

If the number of entries not more than approximately 500, simple sequential scan might be even faster than a hash lookup. Instead of using two QHashes, just use a array or a list to store merchandiseLangStructs and add two methods that use foreach to find the right entry in the list. Then you don't have to synchronize two QHashes when you add/remove entries.

PS: if you decide to go with two QHashes, consider storing merchandiseLangStruct* (pointers) instead of merchandiseLangStruct (values)