How to slow down printing progress values on a label?

I'm using a label to print 0 to 100 as a part of an progress bar in Qt C++. I use the code below to do it but it executes too fast:

for (i = 0; i <= 100; i++)
{
    data = QString::number(i);
    ui->label_29->setText(data + "%");
}

I tried to use sleep() function but it froze the exe file and it couldn't run. I'm thinking of using a thread but I don't know how.

To update the progress bar with a delay between each step without freezing the GUI, you can utilize Qt's QTimer class to schedule the updates at regular intervals.

Here's an example:

#include <QApplication>
#include <QLabel>
#include <QTimer>

class ProgressBarExample : public QObject
{
    Q_OBJECT

public:
    ProgressBarExample() : i(0)
    {
        // Create and configure the label
        label = new QLabel();
        label->setAlignment(Qt::AlignCenter);
        label->setFixedSize(200, 30);

        // Create the QTimer object and connect its timeout signal to the updateProgressBar slot
        timer = new QTimer(this);
        connect(timer, &QTimer::timeout, this, &ProgressBarExample::updateProgressBar);

        // Set the desired interval (in milliseconds) between updates
        int interval = 100; // Adjust this value as per your requirement
        timer->setInterval(interval);

        // Start the timer
        timer->start();

        // Show the label
        label->show();
    }

private slots:
    void updateProgressBar()
    {
        if (i > 100) {
            // Stop the timer if the progress reaches 100%
            timer->stop();
            return;
        }

        QString data = QString::number(i);
        label->setText(data + "%");

        i++; // Increment the counter
    }

private:
    QLabel* label;
    QTimer* timer;
    int i;
};

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    ProgressBarExample example;

    return app.exec();
}

#include "main.moc"

From Qt documentation of QThread:

Managing Threads

>Note: wait() and the sleep() functions should be unnecessary in general, since Qt is an event-driven framework. Instead of wait(), consider listening for the finished() signal. Instead of the sleep() functions, consider using QTimer.

QThread::sleep:
>Avoid using this function if you need to wait for a given condition to change. Instead, connect a slot to the signal that indicates the change or use an event handler...

And since your goal is:

>I'm using a label to print 0 to 100 as a part of a progress bar

Here are 2 ways you can achieve that:

Solution 1:

You can use a QLabel to mimic a progress bar, and make it possible to see it progressing, by using a QTimer, connecting its timeout signal to a lambda that incrementing the value displayed on the label itself.

I used a timeout of 100ms, and I'm looping the progress, you can tweak that to make it stop at a 100 by using QTimer::stop at some condition.

Here's a minimal reproducible example:

#include <QApplication>
#include <QTimer>
#include <QLabel>


int main(int argc,char*argv[])
{
    QApplication a(argc, argv);

    QLabel *l = new QLabel();
    l->setText("0");
    l->setAlignment(Qt::AlignCenter);

    QTimer *t = new QTimer();

    t->connect(t,&QTimer::timeout,[=]()
    {
        l->setText(QString::number((l->text().toInt()+1)%100));
    });

    t->start(100);

    l->show();

    return a.exec();
}

Here's how it looks:

Solution 2:

To display a progress, Qt offers QProgressBar, here's how you can use it in the above solution:

#include <QApplication>
#include <QProgressBar>
#include <QTimer>

int main(int argc,char*argv[])
{
    QApplication a(argc, argv);

    QProgressBar *p = new QProgressBar();

    p->setRange(0,100);

    p->setValue(0);

    p->setOrientation(Qt::Horizontal);

    QTimer *t = new QTimer();

    t->connect(t,&QTimer::timeout,[=]()
    {
        p->setValue((p->value()+1)%p->maximum());
    });

    t->start(100);

    p->show();

    return a.exec();
}

Here's how it looks: