model mapping issue

mzimmers

I have an app build on the model/view framework. Various QWidgets display various parts of the model, and it's always been straightforward; usually I map a column to a QLineEdit or something like that:

    m_mapper->addMapping(ui->lineEditTimeZone, TAG_TIME_ZONE_CODE);

Now, though, I'd like to do something a little more ambitious.

The app interfaces to an embedded application, communicating via XML messages. One of my widgets displays a combo box for selecting time zones. The combo box is populated from this:

    typedef QMap<string, string> TimeZoneMap;
    TimeZoneMap timeZoneMap =
    {
        {"Pacific/Niue",	"NUT+11"},
        {"Pacific/Honolulu",	"HAST+10"},
        {"America/Anchorage",	"AKST+9AKDT,M3.2.0,M11.1.0"},
        {"America/Los_Angeles",	"PST+8PDT,M3.2.0,M11.1.0"},

I want to display the key (eg "Pacific/Niue") to the user, but when I communicate with my embedded app, I use the value (eg "AKST+9AKDT,M3.2.0,M11.1.0"). So, when I initialize my widget, instead of mapping from a column, I have to translate that column from value to key, and map to that. Ideas?

I hope this makes sense; I've been struggling with how to present this issue.

fcarney

Is memory an issue? Because you could map it like you are. Then go through a pull every key/value out and add that back as an inverse map. It will take up more memory, but you only maintain one mapping by hand.

Edit:
This might be more efficient memory wise:
https://stackoverflow.com/questions/21760343/is-there-a-more-efficient-implementation-for-a-bidirectional-map

JonB

@mzimmers
Just checking one thing: you are aware that QComboBoxes can display strings while returning a corresponding data value, which you could populate from your TimeZoneMap? I admit this has nothing to do with QDataMapperWidget, and I haven't seen that support it, so it's possibly quite unsuitable for what you want to do, but just checking?

fcarney

Heh, for fun I wanted to see what it would take to make an efficient dual mapped QMap:

{
        // two way map
        using DualMap = QMap<QString, const QString*>;
        DualMap map;

        auto insert = [](DualMap& map, QString key, QString val){
            auto km = map.insert(key, nullptr);
            auto kmp = &(km.key());
            auto vm = map.insert(val, nullptr);
            auto vmp = &(vm.key());
            *km = vmp;
            *vm = kmp;
        };
        auto retrieve = [](DualMap& map, QString key)->QString{
            return *(map.value(key));
        };

        insert(map, "one","1");
        insert(map, "two","2");
        qInfo() << retrieve(map, "one") << retrieve(map, "1");
        qInfo() << retrieve(map, "two") << retrieve(map, "2");
}

Probably would be better to put this in a generic class though.

mzimmers

Trying to keep the solution understandable to my simple mind, I tried this:

    int index;
    QVariant qv;
    QString qs;
 
    qv = m_modelDevices->getModel()->data(*qmi);
    qs = (qv.toString());
    string k = m_timeZoneMap.key(qs.toStdString());
    it = m_timeZoneMap.find(k);

only to discover that QMap iterators are bidirectional, not random access, so this line is illegal:

index = it - m_timeZoneMap.begin();

So, I'm not sure where to go from here. I could just increment through an interator until I get the one I want, but that seems quite wasteful, even in this day and age of super-fast processors. Maybe a QMap isn't the right container for this after all...?

EDIT: found a solution. It's probably not super CPU efficient, but it works:

    index = distance(m_timeZoneMap.begin(), it);

Seems to work now. Not a perfect solution, and I don't actually use the mapper, but it works. Thanks to all who looked at this.

JonB

@mzimmers
Can you remind me when you even need to map from "extended TZ syntax" value back to "friendly name" column? So far you have only said you have a combo where you want to display the latter but send the former to the embedded app.

mzimmers

@JonB my Qt app is a utility for communicating with an embedded device for status, configuration and the like. The time zone software on the embedded system doesn't support use of the user-friendly strings, which I want to use in my Qt app.

So, the easiest thing seemed to be to present the user-friendly strings (the keys in my map) on Qt, but use the TZ strings (the values) for communication with the embedded system. As an alternative, I could copy my map to the embedded system, but then I'd have to concern myself with field updates, keeping everything in sync, and so on.

JonB

@mzimmers
I understand the embedded cannot handle the friendly names. I understand you need a mapping in the UI app from user friendly names to the tz stuff to send to the embedded app.

What I still don't understand is where/why you need to back from the tz spec in the second column to the friendly name in the first column?

mzimmers

@JonB oh: when I bring up the display below, I populate all the fields with information from the embedded device. In the case of the time zone, it comes in as a TZ string, so I have to convert it to the user-friendly string for display.

JonB

@mzimmers
OK.

So, personally, I'd do it in 5 minutes. I wouldn't bother with any map at all, I don't know how that plays with your data widget mapper or model.

Like I said earlier, QComboBox has addItem(const QString &text, const QVariant &userData = QVariant()) for associated data value against string display, and int QComboBox::findData(const QVariant &data, ...). I would leverage that existing support. I'd just store the friendly name string with the tz-format string as data, and use findData() to back-map.

Is it as efficient at back-mapping as a map would be? No, it's doubtless just a sequential search of the combo items. Of which you have some hundreds? I can't believe that looking through those once when initially populating a combobox is in practice going to lead your app to a grinding halt and upset your users? And you could be getting on with other coding :)

mzimmers

@JonB I agree; your way is simpler. I left the original data as a QMap (didn't feel like changing it), but the combo box population is simpler now:

    // populate the timezone combo box.
    for (it = m_timeZoneMap.begin(); it != m_timeZoneMap.end(); ++ it)
    {
        ui->comboBoxTimeZone->addItem(it.key(), it.value());
    }

Setting the current index:

  // set the timezone combo box index.
    m_modelDevices->getModel()->data(*m_qmi);
    row = m_qmi->row();
    column = TAG_TIME_ZONE_CODE;
    *m_qmi = m_modelDevices->getModel()->index(row, column);
    qv = m_modelDevices->getModel()->data(*m_qmi);
    index = ui->comboBoxTimeZone->findData(qv);
    ui->comboBoxTimeZone->setCurrentIndex(index);

And retrieving the data for sending to the embedded device:

        s = ui->comboBoxTimeZone->currentData().toString().toStdString();
        msg.add(TAG_TIME_ZONE_CODE, s);

Thanks for the suggestion.