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Delete a QString From QStringList



  • Hello everybody,
    imagine i have a QStringList which countains names of persons and i want to delete some names from this QStringList, how can i do it ? i tried
    CodeCountry = "FR";

               QStringList ListName; 
               for (int index =0;index < ListName.length() ; index ++)  {
                       if (ListName[index] == "Jon") || (ListName[index] == "Adam"){
                                  ListName.removeAll(ListName[index]);
               }
    

    but it didn't work, can i get help ? thanks



  • @Zunneh said in Delete a QString From QStringList:

    but it didn't work, can i get help ? thanks

    Why you don't simply do this?

    ListName.removeAll("Jon");
    ListName.removeAll("Adam");
    
    


  • @Zunneh
    You must not remove elements from the list while still iterating through it with the code you show.

    Do as @KroMignon says anyway.



  • @KroMignon's solution is best but if you have to do something similar with a container that doesn't have a convenient function to do it automatically, you could iterator through it in reverse, starting with the last element.



  • @mchinand said in Delete a QString From QStringList:

    you could iterator through it in reverse, starting with the last element.

    Not in this case, given the code as written (which is why I phrased it that way). Even if going in reverse:

               QStringList ListName; 
               for (int index = ListName.length() - 1; index >= 0 ; index --)  {
                       if (ListName[index] == "Jon") || (ListName[index] == "Adam"){
                                  ListName.removeAll(ListName[index]);
               }
    

    If this matches more than one line for any given name (I assume it might, else why use removeAll()), the list will be shortened by more than one element. When you then do the index-- for the next iteration in the for, you could then have an index which is now beyond the new ListName.length(), and ListName[index] will then "crash" (or at minimum revisit already visited elements).

    Your proposal would work for QList::removeAt(index), but not (safely) for QList::removeAll(list[index]).



  • @JonB Right, I didn't read the OP's code closely enough.



  • @KroMignon the example i give was bad

    this is the real example, removing names from a QStringList who finish with the letter ' e ' ( if NameList[index].endswith('e') , so there are many names that finish with' e ', removeAll is applied when you know the names, in my case i don't know the Names, the QStringList will append different Names
    did you understand ?



  • @Zunneh
    In that case, do use the "reverse iteration" (i.e. counting downward) suggested by @mchinand and as per the for loop line I have written above. So long as the remove you have in the if inside it will only remove one row (if (whatever) ListName.removeAt(index)) then it is good.



  • @Zunneh said in Delete a QString From QStringList:

    this is the real example, removing names from a QStringList who finish with the letter ' e ' ( if NameList[index].endswith('e') , so there are many names that finish with' e ', removeAll is applied when you know the names, in my case i don't know the Names, the QStringList will append different Names
    did you understand ?

    That is not what you give as implementation!
    One possible solution could be using QStringList::filter():

    const auto toRemove = ListName.filter(QRegularExpression(".*e"));
    for(const auto &item : toRemove)
        ListName.removeAll(item);
    

    or

    const QRegularExpression filter(".*e");
    for(int idx = 0; idx < ListName.size(); ++idx)
    {
        if(ListName.at(idx).contains(filter))
        {
            ListName.remove(idx);
            --idx;
        }
    }
    


  • @KroMignon yeah after asking the question, i was in the kitchen and it comes on my mind to do indexx -- if he delete one item ( like the second solution )
    on more question, can we put more than one filter ? for example delete name s who finish with 'e', 'y' and 'm' ??

    @JonB i wil try your solution too

    finosh wo

    Thanks guys :)



  • @Zunneh said in Delete a QString From QStringList:

    on more question, can we put more than one filter ? for example delete name s who finish with 'e', 'y' and 'm' ??

    I have not looked to see if accepted, but I would guess either of

    const QRegularExpression filter(".*[eym]");
    const QRegularExpression filter(".*(e|y|m)");
    

    (I assume you mean "finish with 'e', 'y' or 'm'"!). Regular expression [abc] means any one letter of, (ab|c|def) has to be used if multiple letter sequences required.



  • @Zunneh said in Delete a QString From QStringList:

    on more question, can we put more than one filter ? for example delete name s who finish with 'e', 'y' and 'm' ??

    This is a regular expression, so it is up to you to set it up to match your needs.
    For 'e', 'y' and 'm' it would be: const QRegularExpression filter(".*(e|y|m)");
    Take a look at https://www.jrebel.com/sites/rebel/files/pdfs/regular-expressions-cheat-sheet.pdf



  • stl-iterators are safe for calling erase on. The Qt functionality is identical to the one of std::vector.
    Given a generic function bool shouldIDeleteThisString(const QString&) that returns true if the string should be removed from the list you can use:

    for(auto i = list.begin(); i!=list.end();){
        if(shouldIDeleteThisString(*i))
            i=list.erase(i);
        else
            ++i;
    }
    

  • Qt Champions 2019

    @VRonin since you're doing iterator fun:

    list.erase(std::remove_if(list.begin(), list.end(), shouldIDeleteThisString), list.end());
    


  • @Christian-Ehrlicher
    Indeed, because I looked it up, but it doesn't make it readable, or the way it works very intelligible, IMHO! :)


  • Lifetime Qt Champion

    @JonB

    but it doesn't make it readable

    Get used to it, that's the preferred C++ way nowadays ("functional programming").

    Regards


  • Qt Champions 2017

    @aha_1980 said in Delete a QString From QStringList:

    Get used to it, that's the preferred C++ way nowadays ("functional programming").

    The major reason for the mortality rise in the developers demographics ... jumping off a tall building ain't no fun ...


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