How to make a QSlider change with double values

Question

My problem is that I just can't seem to make my QSlider work with double values instead of integer, because I need to make it return double values to a QLineEdit and also set it's own value when I put some value in the edit.

score 10 · Accepted Answer · answered Sep 25 '13 at 14:39

When I was a Qt beginner I started with this tutorial. It is a little bit old (it refers to Qt4.1), but it was good enough to get me started!

I have put together a simple example application that can show you where to start... Maybe you can find it helpful!

#include <QApplication>
#include <QtGui>
#include <QVBoxLayout>
#include <QSlider>
#include <QLabel>

class DoubleSlider : public QSlider {
    Q_OBJECT

public:
    DoubleSlider(QWidget *parent = 0) : QSlider(parent) {
        connect(this, SIGNAL(valueChanged(int)),
            this, SLOT(notifyValueChanged(int)));
    }

signals:
    void doubleValueChanged(double value);

public slots:
    void notifyValueChanged(int value) {
        double doubleValue = value / 10.0;
        emit doubleValueChanged(doubleValue);
    }
};

class Test : public QWidget {
    Q_OBJECT
public:
    Test(QWidget *parent = 0) : QWidget(parent),
        m_slider(new DoubleSlider()),
        m_label(new QLabel())
    {
        m_slider->setOrientation(Qt::Horizontal);
        m_slider->setRange(0, 100);
        QVBoxLayout *layout = new QVBoxLayout(this);
        layout->addWidget(m_slider);
        layout->addWidget(m_label);
        connect(m_slider, SIGNAL(doubleValueChanged(double)),
            this, SLOT(updateLabelValue(double)));
        updateLabelValue(m_slider->value());
    }

public slots:
    void updateLabelValue(double value) {
        m_label->setText(QString::number(value, 'f', 2));
    }

private:
    QSlider *m_slider;
    QLabel *m_label;
};

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

    Test *wid = new Test();
    wid->show();

    return a.exec();
}

#include "main.moc"

score 7 · Answer 2 · answered Sep 25 '13 at 11:36

7

You can simply divide slider value on some constant. For example:

const int dpi = 10; // Any constant 10^n
int slideVal = 57;  // Integer value from slider
double realVal = double( slideVal / dpi ); // float value

answered Sep 25 '13 at 11:36

Dmitry Sazonov

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Well, I've seen this solution before, and I found it's the easiest, but my teacher wanted me to practice signals and slots, so I'd prefer to adding custom signals and slots. But thank you anyway! – Isaac Kennedy Sep 25 '13 at 11:42
@user2814864 your teacher's idea is good as well. You could create your own `QObject` class that will re-translate `QSlider`'s signals, but with `double` arguments. – vahancho Sep 25 '13 at 11:45
@vahancho exactly, but the problem is, I'm still a beginner in both programming and Qt, and I don't have real ideas on how to do that. – Isaac Kennedy Sep 25 '13 at 11:48
@vahancho Ok! I'll give it some shots, but do you know if the documentation could help me with it? – Isaac Kennedy Sep 25 '13 at 11:53
Start with official documentation. Did you read any? - http://qt-project.org/doc/qt-5.0/qtcore/signalsandslots.html – Dmitry Sazonov Sep 25 '13 at 12:12
@DmitrySazonov no, I didn't. Thank you man! I think that this documentation will be very usefull. – Isaac Kennedy Sep 25 '13 at 12:22

How to make a QSlider change with double values

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