Obtaining position (int) of a QMap::iterator

Question

I have written a QListModel based model to access data stored in a QMap. QMap is guaranteed to be sorted. So a conversion from QMap::iterator or const_iterator to int should be possible and sane.

At the moment I decrement the iterator and increment a counter variable:

    QVariantMap::iterator it = m_data.upperBound(name); //or any other iterator
    int pos =  0;
    for (;it != m_data.begin();it--)
        pos++;
    //now pos contains the position of the entry at index in the map

For some data structures there is an int operator-(iterator) defined it seems, allowing int pos = it - m_data.begin(); is there something similar for QMap?

probably not, QMap is likely to be implemented with a binary tree so there is no O(1) operation to get the number of elements between 2 nodes — ratchet freak, Jan 08 '16 at 10:21
I'm actually more concerned about line count than performance (n <= 100), it's a red-black tree btw — Simon Schmeißer, Jan 08 '16 at 10:27

eerorika · Accepted Answer · 2016-01-08T10:39:00.240

3

I'm not familiar with QMap in particular, but typically, that's the simplest algorithm to find the rank of an element in a map.

Assuming Qt iterators are usable with standard algorithms, you can simplify that to std::distance(m_data.begin(), it). Otherwise, you can implement distance for QMap iterators yourself with the algorithm that you showed.

edited Jan 08 '16 at 10:39

answered Jan 08 '16 at 10:33

eerorika

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Obtaining position (int) of a QMap::iterator

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