QML and C++, Float Variable not matching

Olivia Dodonic

Hello :)

I'm using a QObject to send a signal with a float variable to a QML file. But when I print the the value using console.log, the value doesn't match the precise value. For example, the values 116,804 in C++ becomes 116,8041611 and 116,814 becomes 116,8141632. I thought the reason for this might be the conversion from float to real, but as far as I know both, float and real, are stored in IEEE 754 single precision floating point number.

The differences are very small and might not be relevant, but the project I work for requires me to explain why exactly the differences occur and proof that they will stay very small for every value.

Kind regards

mrjj

Hi
Seems just to be a case of how many digits are printed
If you do
qDebug() << qSetRealNumberPrecision( 7 ) << yourfloat;
on c++ side, dont it match what you see with console.log ?

Olivia Dodonic

@mrjj Thank you for your answer, i will try that and report...

Olivia Dodonic

@mrjj Yes, thank you, if I use qDebug() << qSetRealNumberPrecision( 16 ) << yourfloat; I get a simliar value, but the number of digits varies.
For example

std:cout: 0.2
qDebug: 0.2000000029802322
qml: 0.20000000298023224

std:cout: 48.1998
QDebug: 48.19983673095703
qml: 48.19983673095703

Do you maybe know why the number of digits varies?

mrjj

@Olivia-Dodonic
Hi
I think its different default "show values" for how many digits are "printed"
std::cout has
http://www.cplusplus.com/reference/iomanip/setprecision/
to alter it.