How to use QTextStream setprecision

tomy

Hi all,

Consider a small program in which the user each time enters a floating point number less than 1. We want to show that number in a lineEdit with that precision.

This is its code in C++: (just for using setprecision)

   long double d = 0.12345678901;
    string s;
stringstream ss;

ss << setprecision(16);
ss << d;
ss >> s;
    cout << s.length() - 2 << endl;

I tried to use its Qt equivalent like this:

QString s;
QTextStream ss(&s);
long double d = 0.9254178963;

ss << setprecision(16);
         ss << d;
         ss >> s;
         ss << QString::number(d, 'f', s.length());
lineEdit -> setText(s);

I got this error:

'setprecision' was not declared in this scope
ss << setprecision(16);

How to solve it please?

kshegunov

http://doc.qt.io/qt-5/qtextstream.html#qSetRealNumberPrecision

tomy

Thanks for the help.

tomy

This does not work exactly as C++ setprecision(int).

VRonin

This does not work exactly as C++ setprecision(int).

it behaves as: /*stringstream*/ ss << fixed << setprecision(16)

as explained here: https://forum.qt.io/topic/75939/how-to-search-for-a-specific-character-in-a-qstring/26

int guessPrecision(double val){
    double junk;
    int precision = 0;
    for(;!qFuzzyIsNull(std::modf(tester,&junk));++precision)
        val*=10.0;
    return precision;
}

now you can use:
ss->setRealNumberPrecision(guessPrecision(d))

tomy

I'm aware of the prior one. I just was thinking about a better solution.
I used it and it showed 0.000 as result!

 QString s;
 QTextStream ss(&s);
 double d = 0.00001;
 ss << qSetRealNumberPrecision(16);
 ss << d;
 int precision = s.length()-2;
 s.clear();
 ss << QString::number(d, 'f', precision);

By now, qFuzzy is the best choice!

tomy

@VRonin

it behaves as: /*stringstream*/ ss << fixed << setprecision(16)

Yes, unfortunately.