Weird problem with double and comparison operation

pedrogkb

I have a function to check the sum of a table, it has worked so far as it is a very simple function, however I have a very weird issue where the variable soma end up with the value 100, but when comparing it to 100 it returns false.
Here's my code:

    QDoubleSpinBox*  tempSpin;
    double soma = 0;
    for(int i = 0; i< ui->tableSecao->rowCount(); i++){
        tempSpin = qobject_cast<QDoubleSpinBox*>(ui->tableSecao->cellWidget(i, 2));
        soma+=tempSpin->value();
    }
    qDebug() << soma;
    qDebug() << (soma==100);
    if(soma<100){
        return 1;
    }else{
        return 0;
    }

When I put 100 in one row and zeroes on the others, the return of the compatison is true, when I put the actual values, and the sum is exactly 100.00, the return is false.

J.Hilk

@pedrogkb when you compare 2 doubles, or floats for that matter, chances are they are actually not equal

Qt offers QFuzzyCompare for this exact reason
https://doc.qt.io/qt-5/qtglobal.html#qFuzzyCompare

You should read up about double /float in computer science.

pedrogkb

Wow, thanks, I'm a Software Engineering student, i've studied floating point operations but never seem anything about this issue when comparing two values.

mrjj

@pedrogkb
Hi
I like this as friendly info about the issues with floating-point numbers in code.
https://floating-point-gui.de/