QString setnum behavior when inputted value is negative

Dummie1138

What is the expected behavior when the inputted value of a QString is negative, and setnum is operated on it?

JonB

@Dummie1138
str.setNum(-1234); should presumably set the string to "-1234".

Dummie1138

@JonB What about when it is not base 10? For example, in base 16, 1234 is 4D2. Will setNum return "-4D2" or something else?

jsulm

@Dummie1138 Please read documentation: https://doc.qt.io/qt-6/qstring.html#setNum

QString &QString::setNum(int n, int base = 10)

Dummie1138

@jsulm

I have visited this screen before asking this question. Sadly, like my previous visit, I was unable to find a reference to setnum behavior on a non-decimal, negative number. Please let me know if I missed something.

J.Hilk

@Dummie1138 why don't you test ist ?

setting up a simple main.cpp with setNum and qdebug takes less than a minute.

mpergand

@Dummie1138 said in QString setnum behavior when inputted value is negative:

I have visited this screen before asking this question. Sadly, like my previous visit, I was unable to find a reference to setnum behavior on a non-decimal, negative number. Please let me know if I missed something.

Not sure what you really want to do,
if you want to print a negative value in hex with a minus sign in front, you can try this:

QString negHex(int v)
{
    if(v>=0) return QString::number(v,16);
    // negative number
    v=~v+1; // two's complement
    return QString("-%1").arg(v,0,16);
}

examples:

qDebug()<<negHex(1234)<<negHex(-1234)<<negHex(-0x4d2);     ---> "4d2" "-4d2" "-4d2"

Chris Kawa

@Dummie1138 Negative numbers are supported in any base the same. If you pass -1234 in base 16 you will get "-4d2". As @J-Hilk said, it would've taken you less time to just try than post the question.

@mpergand That is absolutely unnecessary. QString's number conversion functions like number and setNum handle minus sign already. No need to reinvent the wheel.

mpergand

@Chris-Kawa said in QString setnum behavior when inputted value is negative:

If you pass -1234 in base 16 you will get "-4d2"

I don't get it ...
QString::number(-1234,16); -->"fffffffffffffb2e"

Chris Kawa

@mpergand Sorry about that. It seems there's more to it. The documentation for the long overload of QString::number says that for bases other than 10 the argument is treated as unsigned. What that means... is kinda ambiguous. The other overloads are not documented :/
It does return "-4d2" on my machine (msvc x64 Qt 6.4.2) for the int overload and looking at the source code it does handle the negative numbers by converting to unsigned and adding - in al bases, so it looks like a bit of a mess.

mpergand

@Chris-Kawa
For me, negative hex values are represented like this:

-1  -->  FF  (8bits)
         FFFF (16 bits)
         etc

It seems things have changed in newer versions of C++ as they allow float representation in hex (strange at first thought to me)

Maybe Qt6 has adopted this new capabilities
versus Qt5 ?

Chris Kawa

@mpergand Qt does not use standard library for number to string conversions, but the source here differs quite a lot from what I see locally in the source downloaded via online installer for Qt 6.4.2. So there seem to be some changes made, I just don't know when exactly, the easiest would probably be check the history in git, but I don't have the repo set up at hand.
What platform, toolset and Qt version are you using?

mpergand

@Chris-Kawa said in QString setnum behavior when inputted value is negative:

What platform, toolset and Qt version are you using?

Qt 5.12.10, OSX 10.14 and Kubuntu 21.04