Modify QtableWidget from a slots

Yoyotl

Hello !
I have a problem that I can't solve :
I have a QTableWidget, and I wan't to modify randomly the text in the rows.
So I have made this :

mainwinow.cpp

    QTableWidget* table = new QTableWidget(this);
    table->setColumnCount(1);
    table->setEditTriggers(QAbstractItemView::NoEditTriggers); 

    for (int i=0; i<1; i++) 
    {
        nb = rand() % 500 + 10;
        table->insertRow(table->rowCount());
        table->setItem(0, 0, new QTableWidgetItem(QString::number(nb)));
    }

and I have a slot in mainwondow.cpp that I call when I press a Button

void MainWindow::changeNbRandom()
{
    int rowToChange = 0;
    rowToChange = rand() % 5;
    int newNb = 5;
    newNb = rand() % 500 + 10;

    table->item(rowToChange, 0)->setText(QString::number(newNb));

But when I press the Button, the application crash.
The debug say that the error is on the last line of the slots.
But if I put the line in dirrectly in mainwindow it works...

Please, help me, I don't understand

JonB

@Yoyotl
Your for loop created/inserted just 1 row. Your table->item(rowToChange, 0) has 0 <= rowToChange < 5. If rowToChange > 0 that's not good.

Also, your table->insertRow(table->rowCount()) inserts row at end, but your table->setItem(0, 0, new QTableWidgetItem(QString::number(nb))); is only ever setting row #0 to have a new QTableWidgetItem, you keep overwriting just that one item.

Yoyotl

Alright, I change that, it was a mistake :

for (int i=0; i<5; i++) /
    {
        nb = rand() % 500 + 10;
        table->insertRow(table->rowCount());
        table->setItem(i, 0, new QTableWidgetItem(QString::number(nb)));
    }

So I have my 5 rows.
I've tested differents sings with the debuggers, and it's seems like I just can't make any action on my QTableWidget table in my slot

Yoyotl

It is possible ? Or how can I do otherwise ?

mrjj

Hi
Yes it possible. and it normally just works

ui->tableWidget_2->item(0,0)->setText("Test");

changes the first row/col of the table widget

Yoyotl

Yeah, I was thinking that too, but it doesn't work...
Debugger sort that :

Christian Ehrlicher

@mrjj said in Modify QtableWidget from a slots:

ui->tableWidget_2->item(0,0)

I would guess this returns a nullptr since there is no QTableWidgetItem set yet - you have to set one before accessing it.

mrjj

Hi
You must assign items to the row/cols or else NULL is returned.
Try with

auto item = ui->tableWidget_2->item(row, 0);
if (item)
    item->setText("Test");
else
    qDebug() << "item is null";

Yoyotl

Itwork's if I put that in mainwindow, but if I put it in my slot like I want to do, the program crash again...

JonB

@Yoyotl
In your slot, put in debugging code to see how many rows/columns/what items your table has, before you try to dereference with table->item(rowToChange, 0). You are using the same table instance as you create with

QTableWidget* table = new QTableWidget(this);

aren't you, because that looks like a local variable.... ? If you have a QMainWindow member variable named table, then statement QTableWidget* table = new QTableWidget(this); is not setting the member one...

mrjj

@JonB
Ahhh good spotted. The dangling pointer classic :)

Yoyotl

Thanks for your answers.
First, I create table in my mainwindow.cpp, and I declare it in mainwindow.h in public.
My slot is in the file mainwindow.cpp
And I can't even test how many rows/columns/item I have in it in my slot, because program crash on a segmentation fault every time that I try to access to table on my slot
Do I need to declare table in antoher place/file ?

mrjj

@Yoyotl

QTableWidget* table = new QTableWidget(this); // this would be wrong as its a local variable

table = new QTableWidget(this); // this would allocate the one you have in the class. (in the .h )

so make sure to remove the first one so there is ONLY the one in the class that you access in the slot

Im 99% sure that is why you crash.

Yoyotl

IT WAS THAT !!
THANK YOU SO MUCH @mrjj !

So yes it was a problem in my declaration of table
That fix all my problems, thank you all !

mrjj

@Yoyotl
Actually it was @JonB that spotted it but
super its fixed :)