Qt Convert Float to ByteArray

neda

Hi,
I used this code for convert float to "ByteArray".
The following codes have different results.
Please guide me.
Thanks

Code 1 (It's Not True):

 float speed=20;
 float calcSpeed= funcCalcSpeed(speed);
 qDebug()<<"calcSpeed: ";
 qDebug()<<calcSpeed;

Output:

calcSpeed:
41.6667

QByteArray byteArray = QByteArray::fromRawData(reinterpret_cast<char *>(&calcSpeed), sizeof(float));
qDebug()<<"byteArray: ";
qDebug()<<byteArray;

Output:

byteArray:
"`\xBF\x96\x02"

Code 2 (It's True):

float val=41.6667f;
QByteArray byteArray = QByteArray::fromRawData(reinterpret_cast<char *>(&val), sizeof(float));
qDebug()<<"byteArray: ";
qDebug()<<byteArray;

Output:

byteArray:
"\xB3\xAA&B"

Christian Ehrlicher

@neda said in Qt Convert Float to ByteArray:

41.6667

Just because your float number has 6 same digits it does not mean it's exactly the same value. That's also the reason why you should not compare two floating point numbers (when they are calculated by different algorithms). See also https://en.wikipedia.org/wiki/Floating-point_arithmetic

sierdzio

Not sure what you are trying to achieve (hint: when posting a thread, also wrinte some question, or at least some "expected result"), but QByteArray has 2 methods to help you work with floats:

Using setNum

const float number = 20;
QByteArray array;
array.setNum(number);

Using number

const float number = 20;
const QByteArray array = QByteArray::number(number);

neda

@sierdzio said in Qt Convert Float to ByteArray:

Not sure what you are trying to achieve (hint: when posting a thread, also wrinte some question, or at least some "expected result"),

Thanks for your reply.
I want edit "Code 1" and have output "Code 2".
Output of "code 2" is true.
But instead of "float val=41.6667f;" I have "funcCalcSpeed(speed)".

Code 2 (It's True):

float val=41.6667f;
QByteArray byteArray = QByteArray::fromRawData(reinterpret_cast<char *>(&val), sizeof(float));
qDebug()<<"byteArray: ";
qDebug()<<byteArray;

Output:

byteArray:
"\xB3\xAA&B"

I tried"setNum" and "number", but they did not have the desired output (Like output of code 2).

Christian Ehrlicher

I still don't get your problem... did you understand what I wrote above?

aha_1980

This post is deleted!

sierdzio

What prevents you from combining both approaches?

const float speed=20;
coinst float calcSpeed= funcCalcSpeed(speed);
const QByteArray byteArray = QByteArray::fromRawData(reinterpret_cast<char *>(&calcSpeed), sizeof(float));
 qDebug()<<"calcSpeed: ";
 qDebug()<<byteArray;

(yeah I think I still don't fully get what you're trying to achieve).

kshegunov

Your problem is not converting the float to a sequence of bytes, but that you assume (erroneously) that 2 floating points are the same if they match up to the 4th decimal digit. You need to read and understand what floating points are and why this assumption can almost never be fulfilled.

(See @Christian-Ehrlicher's link with the wiki article)

neda

@Christian-Ehrlicher
@sierdzio
@kshegunov

Thanks a lot
I understood what is happened and my problem has solved.