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Problem using QList::RemoveAt function

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  • K Offline
    K Offline
    KA51O
    wrote on 26 Jan 2012, 09:58 last edited by
    #10

    [quote author="koahnig" date="1327571719"][quote author="KA51O" date="1327571471"][quote author="koahnig" date="1327571283"][quote author="KA51O" date="1327570403"]I use this:

    @
    while(!list.isEmpty())
    {
    delete list.takeFirst();
    }
    @
    [/quote]

    The condition is missing in your case and
    @
    list.clear();
    @

    would be shorter and probably faster then. [/quote]

    actually list.clear() only removes the objects from the list. It does not delete the objects, which of course is fine if you still use them somewhere else.

    [/quote]

    Sorry, I did not read carefully enough :S I see now.

    Your approach would mean that you store pointers in the list. You delete the objects then, but this would be an endless loop. [/quote]

    No because QList::takeFirst() removes the first item and returns it at the same time.

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    • K Offline
      K Offline
      KA51O
      wrote on 26 Jan 2012, 10:02 last edited by
      #11

      this is Lukas Geyers solution with delete:

      @
      int i = 0;
      while(i < mylist.count())
      {
      if(condition)
      {
      YourType* objToDelete = mylist.at(i);
      mylist.removeAt(i);
      delete objToDelete;
      }
      else
      i++;
      }

      @

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      • K Offline
        K Offline
        koahnig
        wrote on 26 Jan 2012, 10:05 last edited by
        #12

        [quote author="KA51O" date="1327571898"]
        No because QList::takeFirst() removes the first item and returns it at the same time.
        [/quote]
        Correct. Not my day :-(

        Vote the answer(s) that helped you to solve your issue(s)

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        • L Offline
          L Offline
          lgeyer
          wrote on 26 Jan 2012, 10:06 last edited by
          #13

          [quote author="koahnig" date="1327571110"]I admit it is not the most elegant solution and I would vote for your while loop approach. Nevertheless, I do not agree that there be a problem as long as nobodyelse is fidling with the counter i.[/quote]
          Well, it looks like I agree with your disagreement (at least as long as it comes to counting down <code>for</code>) ;-)

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          • I Offline
            I Offline
            Indrajeet
            wrote on 26 Jan 2012, 10:16 last edited by
            #14

            Thanks Lukas While loop works correctly.

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            • G Offline
              G Offline
              goetz
              wrote on 26 Jan 2012, 10:20 last edited by
              #15

              [quote author="koahnig" date="1327568122"]Probably the simplist trick would be to go from the back

              @
              int itotal = mylist.count();
              for(int i = itotal; i > 0; --i)
              {
              if(condition)
              mylist.removeAt(i-1);
              }
              @

              That would be my first check to get it work. I do not work with QList, but there might be better ways to do.

              [/quote]

              I regard the above solution as the most elegant and easy to read one, although I would modify and shorten it a bit more:

              @
              for(int i = mylist.count() - 1; i >= 0; --i)
              {
              if(condition_is_met)
              delete mylist.takeAt(i);
              }
              @

              http://www.catb.org/~esr/faqs/smart-questions.html

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              • I Offline
                I Offline
                Indrajeet
                wrote on 26 Jan 2012, 10:30 last edited by
                #16

                So while loop is best approach is it?

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                • G Offline
                  G Offline
                  goetz
                  wrote on 26 Jan 2012, 10:43 last edited by
                  #17

                  [quote author="Rajveer" date="1327573850"]So while loop is best approach is it?[/quote]

                  Use the solution that works for you and that you understand how it works.

                  http://www.catb.org/~esr/faqs/smart-questions.html

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                  • _ Offline
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                    _Michel
                    wrote on 7 Mar 2014, 13:35 last edited by
                    #18

                    @
                    int i = 0;
                    // size() and count() are identical unless you give a paremeter to count(const T & value)
                    while (i < myList.size())
                    {
                    if (condition is met)
                    {
                    myList.removeAt(i); // "If you don't use the return value, removeAt() is more efficient.":http://qt-project.org/doc/qt-5/qlist.html#takeAt
                    }
                    else
                    {
                    i++;
                    }
                    }
                    @

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                    • K Offline
                      K Offline
                      KA51O
                      wrote on 7 Mar 2014, 14:37 last edited by
                      #19

                      But if you need to call delete then you need to use takeAt. Which is what was discussed above.
                      Anyways your code is just like the one posted by Lukas Geyer in the 3rd post.

                      [quote author="Lukas Geyer" date="1327568215"]@
                      int i = 0;
                      while(i < mylist.count())
                      {
                      if(condition)
                      mylist.removeAt(i);
                      else
                      i++;
                      }
                      @[/quote]

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