Removing rows from a table model

[Perdrix]

If I have a table view that is set for extended selection, I can select disjunct ranges of rows.

If I want to delete them do I have to delete each row individually like this:

QItemSelectionModel* qsm = ui->tableView->selectionModel();
QModelIndexList selectedRows = qsm->selectedRows();

	for (int i = 0; i < selectedRows.size(); i++)
	{
		int row = selectedRows[i].row();
		imageModel->beginRemoveRows(QModelIndex(), row, row);
		imageModel->removeRows(row, 1);
		imageModel->endRemoveRows();
	}

Where my overridden removeRows mf looks like:

    bool ImageListModel::removeRows(int row, int count, const QModelIndex& parent)
        {
            //
            // Remove the rows from our backing data and then call
            // base class removeRows()
            //
            auto first{ std::next(mydata.begin(), row) };
            auto last{ first + (count - 1) };
            mydata.erase(first, last);
            return Inherited::removeRows(row, count, parent);
        }

or is there a better way to do it?

Thanks
David

[JonB]

@Perdrix said in Removing rows from a table model:

I can select disjunct ranges of rows.

"Disjunct" is a big word :) If your range of rows is not contiguous you can only remove them one a time.

And while I think of it: presumably you must be careful for removals after the first one? I am thinking that once one row has been removed it may affect the row numbers of the others to be removed, bear that in mind in your code?

[Perdrix]

Really? I thought disjunct was a much smaller word that dis-contiguous :)

[Perdrix]

@JonB said in Removing rows from a table model:

once one row has been removed it may affect the row numbers of the others to be removed

Oh grief - that's a pain! So clearly I need to revise...

Is this correct - I think it is:

if (Menuitem::remove == item || Menuitem::erase == item)
{
	for (int i = 0; i < selectedRows.size(); i++)
	{
		int row = selectedRows[i].row() - i;
		imageModel->beginRemoveRows(QModelIndex(), row, row);
		imageModel->removeRows(row, 1);
		imageModel->endRemoveRows();
	}
}

If not does anyone have the correct code to do this - my head is starting to hurt!!!

D.

[JonB]

@Perdrix
First, check my logic on this. My thought:

• You start with some selection model containing the indexes/row numbers in the source you want to delete.
• I do not know what order the selection/source row numbers are returned to you in. Suppose it is "random".
• If you delete a low row number first, one at a time, that would cause higher row numbers to move down.
• This would make the row numbers from the remaining selections "wrong". Unless the selection indexes are "live", so that they get decremented for you when a lower numbered row is deleted? You need to check.

The easiest way with deleting numbered rows in anything is to delete downwards in row number order. Then deleting a higher numbered row first does not affect the lower numbered rows. I (think I) would get a list of the source row numbers to delete sorted by row number. Then I would delete those in descending row number order, so I wouldn't have to worry about renumbering?

[Perdrix]

My thoughts are much along the same line as yours

the list of items to remove is in a QModelIndexList which is just a QList<QModelIndex>

I actually came here to ask how to sort one of those in descending order (always assuming the QModelIndex has operator<(), so that I can delete the higher numbered rows first.

I'm a bit concerned by the comment:

bool QModelIndex::operator<(const QModelIndex &other) const
Returns true if this model index is smaller than the other model index; otherwise returns false.
The less than calculation is not directly useful to developers - the way that indexes with different parents compare is not defined. This operator only exists so that the class can be used with QMap.

which sort of suggests I can't do that :(

I'll open a separate question for that.

D.

[JonB]

@Perdrix
You don't need to compare "full indexes". You are only interested in row numbers for your deletions. So you can just sort (e.g. std::sort()?) by QModelIndex::row(), you don't care about columns. And given you have QModelIndexList selectedRows = qsm->selectedRows() I think you have just one entry, with column() == 0, for each row, so no duplicate row numbers.

[Perdrix]

Well, I tried to see what would happen if I ignored the warning, and simply did this:

//
// Sort the list of QModelIndex in descending order so that 
// when we iterate over the list we will delete the rows 
// with higher row numbers first.
//
std::sort(selectedRows.rbegin(), selectedRows.rend());

and it worked perfectly to sort the QModelIndex entries in the list by row number in descending order.

D.

[JonB]

@Perdrix
Umm, isn't this "naughty"?! You do not know what it is sorting by within a QModelIndex, the fact that it "works" by row number is "fortuitous"? Oh, I see, it will use the in-built bool QModelIndex::operator<.

Looking at https://codebrowser.dev/qt5/qtbase/src/corelib/itemmodels/qabstractitemmodel.h.html#83

    Q_DECL_CONSTEXPR inline bool operator<(const QModelIndex &other) const noexcept
    {
           return  r <  other.r
                || ...

you will be OK, but don't tell anyone else :)

[Perdrix]

@JonB I admit I cheated and looked at the code!

D.