Get Value of DoubleSpinBox into a double or float

[tomaschku]

Hello again,
I have a simple calculation:

double a = QVariant(ui->input_a->text()).toDouble();
int b = QVariant(ui->input_b->text()).toInt();
double d = 0;

d = b / (a * a);

ui->text_result->setText(QVariant(d).toString());

When I compile it, no error is shown, but when I edit the Values with the DoubleSpinBoxes, then the Value a doesn't change. (i.e. It's 0.)

Code for checking Values:

ui->text_result->setText("Error! " + QVariant(a).toString() + "|" + QVariant(b).toString());

Before, I used Floats, but because I read, it was better, when I use double I changed the format. (Because of precision, ...)
The Debugger isn't saying anything about this.

[Joel Bodenmann]

QVariant doesn't not do the conversion for you. It's just a means to store any type, but if you store a QString in it, then QVariant::toDouble() will not convert it to a double for you.

Instead, use QString::toDouble():

bool ok = false;
double a = ui->input_a->text().toDouble(&ok);
if (!ok) {
    qDebug("Conversion failed!");
}

Reasons for why QString::toDouble() might fail are simple format issues. You can prevent those by using a QValidator. Alternatively, if your input_a contains composite strings you can use a regex to extract your double.

@tomaschku said in Get Value of DoubleSpinBox into a double or float:

Before, I used Floats, but because I read, it was better, when I use double I changed the format. (Because of precision, ...)

The question is whether you need the precision.

[Joel Bodenmann]

Err... I just read your title... You seem to use QDoubleSpinBox? If so, why not just using QDoubleSpinBox::value() which returns a double? There is no QDoubleSpinBox::text() so I'm not sure if you're mixing things up (or if you're using QDoubleSpinBox::textFromValue() but in any case, if you're using QDoubleSpinBox then using QDoubleSpinBox::value() is the right way to go. If you use something else, such as QLineEdit, then the stuff I mentioned in my first post is the way to go.

[tomaschku]

@Joel-Bodenmann Thank you, for your answer. I'm very new with the UI-Version of C++ and I don't know so much about it... :)

[Joel Bodenmann]

You're welcome - we're here to help :)

Please mark this topic as solved if your questions have been answered.

[kshegunov]

@Joel-Bodenmann said in Get Value of DoubleSpinBox into a double or float:

QVariant doesn't not do the conversion for you.

Just as a note here, QVariant actually does conversions. Whether or not a conversion is possible can be judged by the return value of QVariant::canConvert.

[VRonin]

@tomaschku said in Get Value of DoubleSpinBox into a double or float:

Code for checking Values:
ui->text_result->setText("Error! " + QVariant(a).toString() + "|" + QVariant(b).toString());

when I edit the Values with the DoubleSpinBoxes, then the Value a doesn't change. (i.e. It's 0.)

I think the problem is even worse than what is mentioned above.
double a = ui->input_a->value(); does not imply that when input_a changes it automatically updates a