How to display the result of an operation in incremental fashion strating from 0?

jsulm

@Santhoshpl You did not write what the problem is, but I think I know what happens: you see the result only when the loop finishes, right? Not while it runs. The problem is: the for-loop is blocking the Qt event loop as long as it is running, so the UI will not be updated until the loop finishes.

Or do you mean that you only see the last result? If this is the case than please tell us what ui->product is and what ui->product->display does.

ODБOï

hi
@Santhoshpl said in How to display the result of an operation in incremental fashion strating from 0?:

double num1 = ui->num1->value();
double num2 = ui->num2->value();
double product= num1*num2;
int temp=0;
for(int i = 0; i<product; i++){
temp++;
ui->product->display(temp);

you can use QTimer for that

 double num1 = 10;
    double num2 = 2;
    double product = num1*num2;

    int currentDisplay=0; //<

    QTimer *timer = new QTimer();
    QObject::connect(timer, &QTimer::timeout,[&](){
        if(currentDisplay>=product){
            timer->stop();
        }
        qDebug()<<currentDisplay; // ui->product->display(temp);
        if(currentDisplay<product)currentDisplay++;
    });
    timer->start(50);

Santhoshpl

@jsulm Yes, you are right. I am seeing only the end result after the loop exits.
I want to update the UI as it runs. The ui->product->display(product) displays the result in the UI.

JonB

@Santhoshpl So try @LeLev's suggested code.

Santhoshpl

@LeLev Thank you for the solution, but its crashing.
The error message displayed is:
The program has unexpectedly finished.
The process was ended forcefully.
productApplication.exe is crashed

ODБOï

@Santhoshpl said in How to display the result of an operation in incremental fashion strating from 0?:

crash

can you please show your code ?

Gerd

or..
just add a
QApplication::processEvents();

right after

ui->product->display(temp);

Santhoshpl

@LeLev please find the below code:

void MainWindow::on_controlButton_clicked()
{
double num1 = ui->num1->value();
double num2 = ui->num2->value();
double product = num1 * num2;
double currentDisplay=0;
QTimer *timer = new QTimer();
QObject::connect(timer, &QTimer::timeout,&{
if(currentDisplay>=product){
timer->stop();
}
ui->product->display(currentDisplay);
QApplication::processEvents();
if(currentDisplay<meetingCost)currentDisplay++;
});
timer->start(500);
}
}

JonB

@Santhoshpl
It helps others if you put literal code in this forum's code markers.

Meanwhile, I don't know, @Lelev writes

 QObject::connect(timer, &QTimer::timeout,[&](){

while you write

QObject::connect(timer, &QTimer::timeout,&{

so do you know what you're doing there with your lambda? Is there any reason you change his code for this?

Otherwise run under debugger and look at the context/stack trace when the crash occurs.

Christian Ehrlicher

@JonB said in How to display the result of an operation in incremental fashion strating from 0?:

[&]

Please take a c++ book about lambdas - you're passing by reference and it will therefore go out of scope...

jsulm

@Gerd said in How to display the result of an operation in incremental fashion strating from 0?:

QApplication::processEvents();

This is an ugly work-around and should be avoided. It is better to program in asynchronous way instead of calling processEvents().

Gerd

lol
then you should tell the qt people to remove such examples from the examples in qt sources...

J.Hilk

@Gerd said in How to display the result of an operation in incremental fashion strating from 0?:

then you should tell the qt people to remove such examples from the examples in qt sources...

you're right, it shouldn't be there, can you name/link such an example?

Gerd

Qt\5.12.6.src\qtbase\examples\widgets\dialogs\findfiles
Qt\5.12.6.src\qtbase\examples\widgets\itemviews\pixelator
Qt\5.12.6.src\qtbase\examples\widgets\painting\fontsampler

JonB

@Gerd Can you supply these as clickable links to the on-line docs?

Gerd

heres one of them:
findfiles

Gerd

for these it isnt in the doc pages but in the sourcecode..
pixelator
fontsampler

J.Hilk

@Gerd
Well these examples haven't been touched in years, are very basic but never the less badly designed.

One can redesign those examples, without the need for a processEvent calls, which will make them bigger (source code size).
But IMHO that's something that should be done.

Colins2

@LeLev
I just read your response and it seemed what I need as well.
However, I just get a crash with this code:

I have been having the same problems as the OP (and everyone else, I guess)
This code snippet is just for testing messages written to a QPlainTextEdit widget, nothing else.

void MainWindow::writeLeft()
{
    int32_t cnt = 100;
    QString msg, cno;
    QTimer *ltimer = new QTimer();

    do{
        QTextStream(&cno) << cnt;
        msg = ("Hello left no : "+ cno);
        //message has been constructed, stop the process and display it
        QObject::connect(ltimer, &QTimer::timeout,[&](){
            if(cnt > 0){
                ltimer->stop();
            }
        ui->memo1->appendPlainText(msg);
        });
        cno.clear();
        QThread::sleep(1);
        ltimer->start(10);
        cnt--;
    }while(cnt > 0);
}

If I remove the QTimer statements then I just get the whole lot dumped to the QPlainTextEdit in one go.

JonB

@Colins2
You are re-connecting the timer every time round the loop?