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



    • Ex: Multiplication of two numbers results in 200. I want to display the result from 0 to 200 in an incremental fashion.
      The code which I used is:
    void MainWindow::on_controlButton_clicked()
    {
    
            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);
            }
        }
    

  • Lifetime Qt Champion

    @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.



  • 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);
    


  • @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.



  • @Santhoshpl So try @LeLev's suggested code.



  • @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





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

    right after

    ui->product->display(temp);



  • @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);
    }
    }



  • @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.


  • Lifetime Qt Champion

    @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...


  • Lifetime Qt Champion

    @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().



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


  • Moderators

    @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?



  • 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



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



  • heres one of them:
    findfiles



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


  • Moderators

    @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.



  • @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.



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



  • @Colins2 im not 100% sure what you need to do but see

    
        int32_t cnt = 100;
        QString msg, cno;
        QTimer *ltimer = new QTimer();
    
        //do{
        // QTextStream(&cno) << cnt;
    
        //msg = ("Hello left no : "+ cno);
    
        QObject::connect(ltimer, &QTimer::timeout,[&](){
            cnt--;
            if(cnt == 0){ // <
                ltimer->stop();
            }
            qDebug()<< ("Hello left no : " + QString::number(cnt));
    
        });
    
        cno.clear();
        //  QThread::sleep(1);
        ltimer->start(10);
    
        //   }while(cnt > 0);
    


  • @Colins2
    Glancing only at @LeLev's latest post, this makes much more sense. Give it a go?!



  • @J-Hilk
    I totally disagree with that. What you are doing here is to replace a simple loop routine with an overcomplicated timer driven solution.
    In such simple programs there is absolutly no needance for that.
    Keep it simple and its runs (fast)...



  • @LeLev
    I don't think your solution is what I want.

    What I am trying to achieve is quite simple - or should be.

    I have a PlainTextEdit widget that I am using as a log output.
    At various times through different routines, I want to output a message.
    What happens is that I get no output until the end of the routine, which isn't much use.

    I just cobbled the above routine to try to get just the message output working. It has no real relevance to my project other than the method.
    My loop just created a message with the counter attached and I would expect it to print to the PlainTextEdit widget when called.
    It doesn't do that. Stepping through the code, the statement executes and then the loop continues but nothing appears in the edit field. At the end of the loop it all gets dumped in one go.

    Re. the QTimer, It's new to me. I've never had to use these types of function to get output on the screen.
    As Gerd says in his post, it should be such a simple thing to do but it appears not to be.



  • @JonB
    The idea was to trigger the timer stop so that I could print the current message and then resume.
    I see what you mean though, it shouldn't be necessary to re-connect every time, just start and stop.


  • Lifetime Qt Champion

    @Colins2 said in How to display the result of an operation in incremental fashion strating from 0?:

    At various times through different routines, I want to output a message.

    If I understand you correctly this will not happen in a loop in your main thread - is that correct?
    Is your current implementation with the loop just proof of concept (or an example)?
    As I explained already your loop is blocking the event loop that's why it behaves like it does.
    So, the question is: how are these messages generated and exposed?



  • @Colins2
    It's not just that it shouldn't be necessary, your loop will create a brand new, duplicate connection each time round the loop. You will end up with hundreds of slots on the timer's signal.

    This code really shouldn't be difficult! Though I'm not offering to write, and I'm Python/PyQt anyway. You have to have something which allows the Qt event loop to run if you want to see the output come up as you go along. If your new data arrives from somewhere externally, you might raise a signal and have a slot to update your display. That will be happening within the Qt main event loop, so all will get updated as you go along.

    I see @jsulm is responding more to what you want, I'll leave it to him :)



  • @jsulm
    Yes, it will happen during the loop execution, that is the whole point of my problem.

    Rather than talking abstract things, this is what I want to do:
    I have a large plain text file (300k+) which is an airport database in a flight sim.
    The entries are formatted according to their specs with all entries in plain text to make it easy to read in a text editor if desired.
    There are over 28000 entries in there.
    There are some formatting errors also that stop me from processing the data as I wish.
    I have a program that I wrote in Rad Studio that does exactly what I want.
    It reads every entry, skips the fields I don't want, collects various statistics and file locations that I do want and gives me the output that I am looking for. While processing, it will print whatever I tell it on the screen in a memo field.
    I used this to good effect when trying to get around some of the wrongly formatted data that was crashing my program, i.e. finding out where it was crashing, and on which entry.
    I have got a program that runs error free now and will print out the erroneous entries as they occur so that I can manually edit the file and correct them.

    I have no problem with Rad Studio, I have been a C++ Builder user for years but it only works on Windows. As the sim runs on MacOS and Linux as well, I wanted to try to write the same program that I could compile on all 3 platforms, hence the use of Qt.



  • @Colins2
    At the risk of (a) predicting and (b) infuriating @jsulm (who, btw, does know what he is talking about), I believe he will recommend putting your large file reading code into a separate thread, so that it does not block the UI since it takes so long to read/process. Send send signals from that thread to a slot in the main UI thread which updates the memo field. You will see output as you go along.

    Now, I really hope he does not scream at me, but if you want a rough & ready solution while you await his input, @Gerd's

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

    right after

    ui->product->display(temp);

    will (I think) probably get you by without a separate thread, for this case... !


  • Moderators

    @JonB
    There's always a right way and an easy way to do things.

    alt text



  • @JonB @J-Hilk

    I had thought about it the other way round, i.e. how to create a separate thread for the messages and progress bar but your idea sounds the better way.
    Thanks for everyone's input



  • @Colins2
    In Qt you don't have a choice about this: all UI operations must be performed in the main thread (not sure how that's defined, maybe the one where you call QApplication from), any non-UI can be performed in separate threads, but not the other way round.

    Hence in Qt: "All threads are equal, but some are more equal than others" :)



  • @JonB
    Thanks :-)
    Just reading through the Mandelbrot code; that seems to be the sort of thing I want.


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