Problem with real values

[clostridium_difficile]

Hello, I divide two integer numbers like this:

qreal scaleFactorX = ui->workSpace->width() / width;

but in my application it equals 0 (zero!). qDebug prints 0 and debbuger shows it is 0. How to repair it, whats wrong with this code?

[Christian Ehrlicher]

@clostridium_difficile said in Problem with real values:

but in my application it equals 0

Everything else would surprise me - C basics. Doing arithmetics with integer values will result in an integer value.

[clostridium_difficile]

@Christian-Ehrlicher said in Problem with real values:

@clostridium_difficile said in Problem with real values:

but in my application it equals 0

Everything else would surprise me - C basics. Doing arithmetics with integer values will result in an integer value.

Well, dividing two integer values and assgning result to qreal, should be casted to double, shouldn't it?

[Christian Ehrlicher]

@clostridium_difficile said in Problem with real values:

should be casted to double, shouldn't it?

For sure, the result of the aritmetic will be casted to the left hand side type.

[mrjj]

Hi
Beware of dragons

qreal scaleFactorX = 10 / 3; == 3

qreal scaleFactorX = 10 / 3.0 == 3.33333;

qreal scaleFactorX = static_cast<float>(10) / 3; == 3.33333;

the reason is that 2 ints will use the integer division version of operator / so
even when you assign it to a double the precision is lost.

[JonB]

@clostridium_difficile said in Problem with real values:

qreal scaleFactorX = ui->workSpace->width() / width;

qreal scaleFactorX = ((qreal)ui->workSpace->width()) / width;

[J.Hilk]

@JonB
uuh, c-style cast 😨

still an upvote as you're technically correct, the best kind of correct.

[JonB]

@J-Hilk
Oh dear, damn! Really, you want me to write:

qreal scaleFactorX = static_cast<qreal>(ui->workSpace->width()) / width;

is that right?

Oh, now I see that's effectively what @mrjj wrote at the end of his examples, I hadn't noticed, sorry.

[kshegunov]

@JonB said in Problem with real values:

is that right?

As we are long done with this question anyway, you could use constructor-style cast instead:

qreal scaleFactorX = ui->workSpace->width() / qreal(width);

(moved it to the end for readability)

[JonB]

@kshegunov
Indeed you could. All this fuss effectively over qreal(width) versus (qreal)width ;-)

[kshegunov]

@JonB said in Problem with real values:

All this fuss

There's no fuss. They're exactly the same - a C-style cast.

[JonB]

@kshegunov said in Problem with real values:

They're exactly the same - a C-style cast.

Oh dear! I thought we went through this a while ago. You don't really mean they are exactly the same, do you? Only because it's qreal here? If it were ClassName(variable) it would be an instance creation instead of a cast, wouldn't it?

[kshegunov]

@JonB said in Problem with real values:

I thought we went through this a while ago.

We may've, I don't remember ... and I'm bored so I have nothing better to do than to argue uselessly ... :)

If it were ClassName(variable) it would be an instance creation instead of a cast, wouldn't it?

This is where C++ really shines ... in its stupidity. It's going to be trying a cast if it can't match a suitable constructor, but if there is a matching constructor you're going to get an anonymous object ... fun, ain't it? And compilers differ in their interpretation ... I've had this:

QMutex x;
{
   QMutexLocker(&x);
}

generate "can't cast to QMutexLocker" error with MSVC, but fly seamlessly through gcc ... that language, go figure ...

[JonB]

@kshegunov said in Problem with real values:

This is where C++ really shines ... in its stupidity. It's going to be trying a cast if it can't match a suitable constructor, but if there is a matching constructor you're going to get an anonymous object ... fun, ain't it?

Ah ha! That at least explains it, a bit :) Phew! I thought you were going to shoot me down!

What I had in mind is: when I see (Something)something I just know it's a cast (yes, static_cast<> is better). But when I see Something(something) am I looking at a function call, an initializer, a cast, even (yikes!) a macro, ...? So personally I wouldn't use that one for a cast, it has enough other uses already!