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How to convert string to double ?

  • Hi all,

    I am new in qt. I want convert string to double . My string is= "12345.54". When i used string.toDouble() i am getting this result = 12345.5 , i want this result=12345.54, why i can't this result ? Also when i changed string this = "12345.56" or "12345.57", i am getting this result=12345.6


  • Lifetime Qt Champion


    How do you determine that you have that ?

  • My code is below:

    QString number="12345.54";
    double x=number.toDouble();

    My output is= 12345.5

  • @Mucahit
    That is just an artefact of how qDebug() displays a double by default (6 digits). use QString QString::arg(double a, int fieldWidth = 0, char format = 'g', int precision = -1, QChar fillChar = QLatin1Char(' ')) const to specify more precision.

  • Lifetime Qt Champion

    You can use qSetRealNumberPrecision to ensure you have the precision you want to show the numbers you are using.

  • @SGaist
    I didn't know about your way. How do you access qDebug()'s QTextStream to call yours? Are you saying you just call qDebug().qSetRealNumberPrecision()??

  • Lifetime Qt Champion


    double testValue = 3.141592653589;
    qDebug() << "Some text" << testValue;
    qDebug() << "Some text" << qSetRealNumberPrecision(12) << testValue;

    The second does not show the full extent of testValue but illustrates the use.

    Note that like with any floating point number, the representation is not guaranteed as not every number can be represented in a computer.

  • @SGaist
    Hmm. Thank you. I find this use of C++ streams (and <<) hard to get my head around, logically! If you can go qDebug() << value to print out a value, I find it conceptually difficult to send qSetRealNumberPrecision(12) to the operator in the same way.

    And while we're here: does that << qSetRealNumberPrecision(12) only affect items output anywhere in the same single statement? Or just the item immediately to the right of it? Or is it permanently altering the output format of the qDebug() stream from then onward? Or...? :)

  • @JonB
    It affects everything to the right, in the same statement. If my test is correct.

     QString testValue = "3.141592653589";
        double pi = testValue.toDouble();
        qDebug() << "by default" << pi;
        qDebug() << "in the same statement witout qSetRealNumberPrecision() "  << pi << "and after setting qSetRealNumberPrecision() "<< qSetRealNumberPrecision(12) << pi << " one more " << pi;
        qDebug() << "after" << pi;
    by default 3.14159
    in the same statement  witout qSetRealNumberPrecision()  3.14159 and after setting qSetRealNumberPrecision()  3.14159265359 one more 3.14159265359
    after 3.14159

  • Thank you so much. I never thought the mistake was caused by qDebug(), it really works. So how can i print this value on any label ? i want thousand separator. For example:

    QString number="12345.54";
    double x=number.toDouble();
    qDebug()<<"result"<<qSetRealNumberPrecision(12)<<x; //this output correct


    I still have the same problem. I want it to write like this at label = "12,345.54" . How can I do that ?

  • @Mucahit
    I already told you in the first place! See the link in my first reply to your question.
    That overload also offers to deal with locale, for your desired thousand-separator.
    You can use that with qDebug(), QLabel::setText() or anywhere else a string is wanted.

  • No, I guess I could not explain exactly. The problem is not printing on labels right now.

    QString number="12345.54";
    double x=number.toDouble();
    qDebug()<<"result"<<qSetRealNumberPrecision(12)<<x; //this output correct

    but this output is not correct :

    qDebug()<<"result"<<qSetRealNumberPrecision(12)<<QString("%L1").arg(x); //this output not correct

    This is the exit I got : some text 12,345.5
    This is the exit I want: some text 12,345.54

  • Lifetime Qt Champion

    This post is deleted!

  • @Mucahit
    You are simply mixing up qSetRealNumberPrecision() with QString.arg() and trying to combine them in a way which will never work.

    Please get rid of qSetRealNumberPrecision(), pretend you had never heard of it, it only affects qDebug(), you won't be able to use for setting your label text. Just use the function I linked you, nothing else, look at all of its arguments, there is even an example there.

  • Yes I looked but I couldn't because I'm new to the qt. I know you're mad at me but I couldn't look at the documents. Can you give me a sample code? How can I divide the string number "12345.54" by thousands ? I want to get this result = 12,345.54

  • Moderators


    int main(int argc, char *argv[])
        QApplication a(argc, argv);
        auto *l = new QLabel();
        const double d{12345.54};
        const QString str = QLocale(QLocale::German).toString(d, 'g',12);
        qDebug() << str;
        return a.exec();

  • Ohh, it really works. I apologize for bothering you. I am changing the topic to solved. Thank you so much !!!

  • @Mucahit said in How to convert string to double ?:

    I know you're mad at me

    I was never mad at you! :)

    Unfortunately for your usage beyond just with qDebug() (because you want to be able to get a string to put in a widget), qSetRealNumberPrecision() was a red herring (means: not relevant here), and was confusing you. A string solution, like QString::arg() or @J-Hilk's QLocale::toString(), which allows you to request the number formatted to a locale (thousand separator character, decimal point character) and to a specified precision (more than the default 6 digits) is what you want.

  • I understand everything much better now. Thank you again for everything sir.

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