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How to delete a row of a QGridLayout



  • Hello,
    I have to make a button able to delete the last row of a QGridLayout. Sure, I used signals and slots but I don't arrive to create a functional slot.
    In addition, I want that the user is able to add row at this Grid (I succeed for that) but he can't add an unlimited number (I use an if condition and count the number of row of my grid).
    So I tried to use the hide method but after many delete the user can't add rows. I also tried with takeAt(), removeWidget(), removeItem().

    Thanks a lot if you have an idea to do that. (I'm in begginer in QT)


  • Lifetime Qt Champion

    Please show us what you did.



  • @maxime0601

    What do you mean by "deleting a row"? You have to remove the Widgets / Items from your layout at that position. A layout is defined by its widgets / items.

    You could delete / remove all items in a specific row, but there is no layout-"row" that you can delete.



  • void MyGridBox::removeRow(void){
        int row = this->rowCount();
        if (row > 6)
        {
            QLayoutItem *child; //Taken from takeAt(int) documentation
            while ((child = this->takeAt(row)) != nullptr) {
                child->widget()->hide();
                delete child;
            }
    //        for (int i(0); i > 4 ; ++i){
    //            this->itemAtPosition(row,i)->widget()->hide();
    //            delete this->itemAtPosition(row,i);
    //        }
        }
    }
    

    @Pl45m4 I want to remove the widgets of the row but the row doesn't disappear and I can't explain why. If I add some rows after deleting one, the limit of the number of rows change.

    @Christian-Ehrlicher the part which are in commentary is an another try. And the other part is inspired of the doc and it was the better result of my tries.



  • @maxime0601

    int row = this->rowCount();
    ...
    while ((child = this->takeAt(row)) != nullptr) {
    

    Don't rows (for takeAt(row)) count from 0 to rowCount() - 1? So your code will do nothing (docs: "If there is no such item, the function must do nothing and return 0."), and never enter the while loop?

    As for the commented-out attempt:

    //        for (int i(0); i > 4 ; ++i){
    //            this->itemAtPosition(row,i)->widget()->hide();
    //            delete this->itemAtPosition(row,i);
    //        }
    

    won't do what you intend because you delete at i and then increment i, not taking into account that you have done a deletion which squeezes other items down (i.e. it deletes every other item)? That apart, i starts at 0 and your loop while condition is i > 4, so it will never enter the loop anyway.

    In both these cases you would find out what is/is not happening if you put some kind of trace/debug statements into your code.



  • @jonb
    For the part with the takeAt(row), I think that I haven't really understand the method. However, it works but it delete more than I hoped.

    For the other part, the i > 4 is just a mistake when I edited my code but with i < 4 the program crashs. If I understand well your explain I just have to delete the first element and after it will delete others under it?



  • @maxime0601 said in How to delete a row of a QGridLayout:

    I edited my code but with i < 4 the program crashs

    i < 4 only works if you have 4 items in your Grid, otherwise there is no column 3.

    You have to replace the 4 with the number of your current cols.



  • @pl45m4

        int row = this->rowCount();
        int col = this->columnCount();
        if (row > 6)
        {
            for (int i(0); i < col ; ++i){
                this->itemAtPosition(row,i)->widget()->hide();
                delete this->itemAtPosition(row,i);
            }
        }
    }
    

    It also crashs like this.



  • @maxime0601
    I already said/suggested

    int row = this->rowCount();
    ...
    this->itemAtPosition(row,i)->widget()->hide();
    

    Don't rows count from 0 to this->rowCount() - 1? So your row exceeds the number of rows, hence crashes? Test what this->itemAtPosition(row, 0) returns, or crashes?

    Then when you have this right, I don't think you should directly delete an item (which is still owned by the grid layout), shouldn't you do some kind of:

    delete this->takeAt(?)
    


  • @maxime0601

    Try to remove the widget from your layout (instead of hiding it).



  • @jonb It continues to crash

    @Pl45m4 I also tried with

    this->removeWidget(itemAtPosition(row,0)->widget()->hide();
    

    and it crashes too.


  • Lifetime Qt Champion

    @maxime0601 said in How to delete a row of a QGridLayout:

    It continues to crash

    then please use the debugger to see what exactly happens and where...



  • @maxime0601

    @jonb It continues to crash

    @Pl45m4 I also tried with

    this->removeWidget(itemAtPosition(row,0)->widget()->hide();

    and it crashes too.

    Last time of saying: what does itemAtPosition(row,0) return/do, given your value of row?? How difficult is it to test that?



  • @jsulm I have a signal of segmentation fault. It confirms that I try to use an outside element of my grid


  • Lifetime Qt Champion

    @maxime0601 said in How to delete a row of a QGridLayout:

    I try to use an outside element of my grid

    Well, then fix that. And I know that it is crashing and my suggestion to use debugger is still valid: you will see where exactly it is crashing and what the values of variables at that time.
    Did you read what @JonB wrote?



  • @jonb I'm sorry if I don't understand what you mean but when I tried itemAtPosition(row,0) it crashes.
    @jsulm the problem is at column 4 and row 7 and it is the position of one of the element that I want to delete.

    Oh I tried with row - 1 and it seems works



  • @maxime0601

    @jonb I'm sorry if I don't understand what you mean but when I tried itemAtPosition(row,0) it crashes.

    Which is why I said your program crashes all the time! You are not understanding that rows are numbered starting from 0 and going to rowCount() - 1, not to rowCount(). That's how most arrays etc. are numbered in C/C++. I don't know how to explain any better, you really need to understand this fundamental, rather than banging stuff in without understanding:

    Oh I tried with row - 1 and it seems works

    You really need to understand why that it is, rather than "seems to work".



  • int row = this->rowCount();
    int col = this->columnCount();
    if (row > 6)
    {
       for (int i(0); i < col ; ++i)
       {
             this->itemAtPosition(row - 1,i)->widget()->hide();
             delete this->takeAt(row - 1);
       }
    }
    

    So, when I used this code, the last row disappeared as I want but the disposition of the others elements in the grid changes.

    @JonB yes I knew this for vector but I totally forgot it. I'm sorry for that.



  • @maxime0601
    I'm sorry but it's more complex than you are expecting. You really need to read https://stackoverflow.com/questions/5395266/removing-widgets-from-qgridlayout. I know it looks a lot more involved than what you are trying, but there you are....



  • @jonb Thanks a lot, I've searched but I don't found similar topic. I think that I can succeed with this.
    Thanks a lot everyone for your help


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