QMap, index of entry without iterating through

[SPlatten]

I have an instance of QMap, I need to get the index of an entry in the map. Of course I could iterate through the map until I find a match.

[KroMignon]

@SPlatten said in QMap, index of entry without iterating through:

o I then iterator through keys? I was hoping for something where I could supply a key to look up and it returns the index of the entry? As I said this is easy enough to write, just didn't want to do it if its already available.

I am not sure to understand what you want to do :(

A QMap() is an ordered list, so each time you add a new key, index of each key may change. Is QMap() the right type for you need, perhaps QHash() may be better?

[J.Hilk]

@SPlatten what do you mean with index ? the key ?

https://doc.qt.io/qt-5/qmap.html#keys

[SPlatten]

@J-Hilk each entry in the map is keyed with the host name for a particular machine, I need to translate this into a position in the map, because when entries are added to the map controls on the display are also added using an index, so the position in he map is important so I can lookup the correct control to update.

[J.Hilk]

@SPlatten If I understand you correctly, then keys() is still the function to call.

it should be ordered by the order of insertion into the QMap, except duplicated keys of course

[SPlatten]

@J-Hilk, do I then iterator through keys? I was hoping for something where I could supply a key to look up and it returns the index of the entry? As I said this is easy enough to write, just didn't want to do it if its already available.

The other part of this problem:
https://forum.qt.io/topic/131333/ui-finding-control-by-using-literal-string

Once I have the key I want to use it to construct a control name, but how do I find the control using a string?

[J.Hilk]

@SPlatten

myMap.keys().indexOf("someKey") ?

[artwaw]

What if instead of QMap you'd go with QVector of QPair? Seems like more convenient for what you need.

[KroMignon]

@SPlatten said in QMap, index of entry without iterating through:

o I then iterator through keys? I was hoping for something where I could supply a key to look up and it returns the index of the entry? As I said this is easy enough to write, just didn't want to do it if its already available.

I am not sure to understand what you want to do :(

A QMap() is an ordered list, so each time you add a new key, index of each key may change. Is QMap() the right type for you need, perhaps QHash() may be better?

[SPlatten]

@KroMignon , thank you, will use QHasp.

[KroMignon]

@SPlatten said in QMap, index of entry without iterating through:

are you saying the index order can change with each new insertion into the map?

Yes I do (cf. https://doc.qt.io/qt-5/containers.html):

QMap<Key, T> This provides a dictionary (associative array) that maps keys of type Key to values of type T. Normally each key is associated with a single value. QMap stores its data in Key order; if order doesn't matter QHash is a faster alternative.

[JonB]

@J-Hilk said in QMap, index of entry without iterating through:

@SPlatten If I understand you correctly, then keys() is still the function to call.
it should be ordered by the order of insertion into the QMap, except duplicated keys of course

For the record, you should not say this. QMap is not ordered by insertion, it is ordered by keys. And keys() (or values()) comes out in ascending key order. Unless perchance you are talking about the ordering of the values in a multi-map, where within one key the values are ordered by insertion, but I don't think this is what you/OP meant.

@SPlatten
Like others, I am confused about what you are storing in the map, and what you are using the values for. And again as others have said, if for some reason you care about an index number, you should not be using a QMap.

but how do I find the control using a string?

QObject::findChildren<optionalWidgetType>(optionalWidgetObjectName). Pick whatever is best to start searching down from as the ancestor widget you call findChildren<>() on. (findChild<>() uses this to pick out just one.)

[SPlatten]

@JonB , as suggested by @KroMignon I am now using QHash, I store the host name of remote systems int the map and use the index of there position to select controls that are in a display.

I'm now using findChild

[JonB]

@SPlatten said in QMap, index of entry without iterating through:

and use the index of there position

Not sure what you mean by this, though if it's working for you it doesn't matter. I think/assume you are storing some index number as the value against a key in a QHash.

[J.Hilk]

@JonB said in QMap, index of entry without iterating through:

For the record, you should not say this. QMap is not ordered by insertion, it is ordered by keys.

I never said, QMap us ordered by insertion!

I said:
>keys() is still the function to call. it should be ordered by the order of insertion into the QMap

it being here the QList<Keys> that keys() returns.

---

nope, was barking up the wrong tree. @JonB is right

[JonB]

@J-Hilk
As I said, QList<Key> QMap::keys() const (and for that matter values() too) is ordered by ascending key, not by ascending order of key insertion.

Returns a list containing all the keys in the map in ascending order.

Where do you quote from which says anything about insertion order? QMap does not maintain any insertion order.

[J.Hilk]

@JonB
I said, should.

And because of the why the template is set up, it iterates over the map

1030 template <class Key, class T>
1031	Q_OUTOFLINE_TEMPLATE QList<Key> QMap<Key, T>::keys() const
1032	{
1033	    QList<Key> res;
1034	    res.reserve(size());
1035	    const_iterator i = begin();
1036	    while (i != end()) {
1037	        res.append(i.key());
1038	        ++i;
1039	    }
1040	    return res;
1041	}

so as long as you don't have multiple values to one key,

except duplicated keys of course

then QMap should be a very linear tree and keys should return in our expected order.

Everything with caveats of course, this all breaks very easily :D

[JonB]

@J-Hilk
I really do not understand what you are saying. That const_iterator i = begin(); is iterating the QMap in ascending key order, and so that is what keys() returns, not insertion order. I have taken this to a PM post to you to clarify, rather than hijacking this thread.

[J.Hilk]

Disregard almost everything I said 🙈,
of course the internal storage is sorted on key/value pair insert