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Put QList items inside a QMap

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  • M Offline
    M Offline
    Mr Gisa
    wrote on last edited by
    #1

    I was wondering if it's possible to add a QList<QString> values inside of a QMap<QString, int> without the need to loop over QList. Is there a Qt or C++ std API for that? All the ints will be false.

    JonBJ 2 Replies Last reply
    0
    • M Mr Gisa

      I was wondering if it's possible to add a QList<QString> values inside of a QMap<QString, int> without the need to loop over QList. Is there a Qt or C++ std API for that? All the ints will be false.

      JonBJ Offline
      JonBJ Offline
      JonB
      wrote on last edited by JonB
      #2

      @Mr-Gisa
      (I'm sure you can save a tiny bit of code via a std:: method.) But does it really matter? Isn't the time taken to loop through the list trivial, at least compared to creating the items in the map, which ultimately I presume must be done one at a time?

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      • M Offline
        M Offline
        Mr Gisa
        wrote on last edited by Mr Gisa
        #3

        Actually it doesn't matter, I'm just curious, I was wondering and decided to ask.

            for (const auto &item : qAsConst(items)) {
                mItems.insert(item, false);
            }
        

        That is how I'm doing it.

        JonBJ Gojir4G 2 Replies Last reply
        0
        • M Mr Gisa

          Actually it doesn't matter, I'm just curious, I was wondering and decided to ask.

              for (const auto &item : qAsConst(items)) {
                  mItems.insert(item, false);
              }
          

          That is how I'm doing it.

          JonBJ Offline
          JonBJ Offline
          JonB
          wrote on last edited by JonB
          #4

          @Mr-Gisa
          That seems fine! Ultimately there's not going to be a "create many map keys from many input keys all in one go" low-level functionality, so you will still have to iterate through the inputs.

          What you could do is if your input list is sorted by key already, look at iterator QMap::insert(const_iterator pos, const Key &key, const T &value) to save insertion look-up time. However, if your input list is not already in sorted order, it won't be worth sorting it first! :)

          There's also QMap::QMap(std::initializer_list<std::pair<Key, T> > list), but I don't think you start with a suitable initializer list format....

          1 Reply Last reply
          1
          • M Mr Gisa

            Actually it doesn't matter, I'm just curious, I was wondering and decided to ask.

                for (const auto &item : qAsConst(items)) {
                    mItems.insert(item, false);
                }
            

            That is how I'm doing it.

            Gojir4G Offline
            Gojir4G Offline
            Gojir4
            wrote on last edited by
            #5

            @Mr-Gisa You may consider using only one QMap, except if you need to keep insertion order of your QList. Then you can access the list when needed using QMap::keys().

            1 Reply Last reply
            0
            • M Mr Gisa

              I was wondering if it's possible to add a QList<QString> values inside of a QMap<QString, int> without the need to loop over QList. Is there a Qt or C++ std API for that? All the ints will be false.

              JonBJ Offline
              JonBJ Offline
              JonB
              wrote on last edited by
              #6

              @Mr-Gisa said in Put QList items inside a QMap:

              All the ints will be false.

              BTW, if this is always the case, you don't need a QMap and a QHash will be quicker/smaller.

              1 Reply Last reply
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              • M Offline
                M Offline
                Mr Gisa
                wrote on last edited by
                #7

                @Gojir4 said in Put QList items inside a QMap:

                QMap

                What you mean with using only one QMap? I'm using only one QMap already.

                @JonB

                BTW, if this is always the case, you don't need a QMap and a QHash will be quicker/smaller.

                Yes, it's a QMap<QString, bool>. In this case is better to use a QHash? Why?

                Gojir4G 2 Replies Last reply
                0
                • M Mr Gisa

                  @Gojir4 said in Put QList items inside a QMap:

                  QMap

                  What you mean with using only one QMap? I'm using only one QMap already.

                  @JonB

                  BTW, if this is always the case, you don't need a QMap and a QHash will be quicker/smaller.

                  Yes, it's a QMap<QString, bool>. In this case is better to use a QHash? Why?

                  Gojir4G Offline
                  Gojir4G Offline
                  Gojir4
                  wrote on last edited by
                  #8

                  @Mr-Gisa What I mean is to replace the QList with QMap (or QHash)

                  1 Reply Last reply
                  0
                  • M Offline
                    M Offline
                    Mr Gisa
                    wrote on last edited by
                    #9

                    The QList is an argument of a method, I don't have two containers in the class.

                    1 Reply Last reply
                    0
                    • M Mr Gisa

                      @Gojir4 said in Put QList items inside a QMap:

                      QMap

                      What you mean with using only one QMap? I'm using only one QMap already.

                      @JonB

                      BTW, if this is always the case, you don't need a QMap and a QHash will be quicker/smaller.

                      Yes, it's a QMap<QString, bool>. In this case is better to use a QHash? Why?

                      Gojir4G Offline
                      Gojir4G Offline
                      Gojir4
                      wrote on last edited by
                      #10

                      @Mr-Gisa said in Put QList items inside a QMap:

                      Yes, it's a QMap<QString, bool>. In this case is better to use a QHash? Why?

                      From the doc of QMap:

                      QMap and QHash provide very similar functionality. The differences are:

                      • QHash provides average faster lookups than QMap. (See Algorithmic Complexity for details.)

                      • When iterating over a QHash, the items are arbitrarily ordered. With QMap, the items are always sorted by key.

                      • The key type of a QHash must provide operator==() and a global qHash(Key) function. The key type of a QMap must provide operator<() specifying a total order. Since Qt 5.8.1 it is also safe to use a pointer type as key, even if the underlying operator<() does not provide a total order.

                      1 Reply Last reply
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                      • M Offline
                        M Offline
                        Mr Gisa
                        wrote on last edited by
                        #11

                        Thank you guys, helped a lot.

                        JonBJ 1 Reply Last reply
                        0
                        • M Mr Gisa

                          Thank you guys, helped a lot.

                          JonBJ Offline
                          JonBJ Offline
                          JonB
                          wrote on last edited by JonB
                          #12

                          @Mr-Gisa
                          I was actually going to say:

                          If you have nothing useful to store for the "value" part of a QHash/QMap, your case could actually suffice with a QSet. The advantage being (given that QSet is only really hiding a QHash anyway), there is a QSet<T> QSet::fromList(const QList<T> &list) (http://doc.qt.io/qt-5/qset.html#fromList) static method which would satisfy for sure your adding of all the strings in your list without you having to write any iteration code/function :)

                          1 Reply Last reply
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