Convert QbyteArray to qreal

dziko147

Hello ,
Iwant to convert a QbyteArray to qreal .

I tried many solution but usually i get value = 0 ;

I read in documentation that i should use a QDataStream .

Does Anyone know how to solve this problem :)

mrjj

Hi
Its impossible to guess for anyone as it really, depends on what is inside the QbyteArray.
If its related to the QCanBusFrame thing, then I suggest trying to dump the payload and see what is inside.
Only you can know.

We cannot convert some random amount of bytes to a qreal as it would not be correct.

Does to int give a valid value ? Are you sure ther eeven IS a qreal value send from the other end ?

dziko147

@mrjj yes its a payload of a Can bus frame .
the can bus send 5byte .
And the type is real .

Christian Ehrlicher

@dziko147 said in Convert QbyteArray to qreal:

5byte .
And the type is real .

No. A standard c real value is either 4 or 8 bytes. If you get 5 bytes you have to interpret them by your own based on the spec from your sending device.

J.Hilk

@dziko147 said in Convert QbyteArray to qreal:

@mrjj yes its a payload of a Can bus frame .
the can bus send 5byte .
And the type is real .

Great, only 120 possible permutations.

You'll need more information from the sender side.

dziko147

the frame sent is compouned of 1byte of bus status and then 4 byte for the data (the real variable) .
So in this case what should I do ?

J.Hilk

@dziko147 there are tons of ways,

I personally would prefer QDataStream
Quick and dirty:

//QByteArray frame;
    float value{0};
    memcpy(&value, frame.data()+1, 4);

50/50 that this would result in the "correct" float

dziko147

@J-Hilk thank you
Can you please provide an example with QDataStream :)

JonB

@J-Hilk said in Convert QbyteArray to qreal:

memcpy(&value, frame.data()+1, 4);

OOI, would you really write 4 as you have done for the size? (I admit I know nothing about what a bus-frame is!)

Christian Ehrlicher

@JonB said in Convert QbyteArray to qreal:

OOI, would you really write 4 as you have done for the size?

Because of 'dirty' :D

KroMignon

@dziko147 said in Convert QbyteArray to qreal:

Can you please provide an example with QDataStream :)

It is so hard to read documentation?

//QByteArray frame

QDataStream stream(frame);

quint8 byte;
float value;
stream >> byte; // skip first byte
stream >> value: // read float value

Christian Ehrlicher

And only one of the two examples will give the 'correct' real value :D

J.Hilk

@JonB you will have noticed, I omitted also any kind of size checks as well :D

@dziko147 ,@KroMignon gave a good example

if you do not care about the status byte, you could also use QDataStream::skipRawData,

just saying :D

dziko147

Still not working :/ :/
I get this value

     1.36179e+30

dziko147

Currently I test a static QByteArray .
this is my code :
QByteArray rpmdata = "FF10001000100010001000100010001000";
qDebug() << rpmdata << "this is data" ;

QDataStream stream(rpmdata);

quint8 byte;
qreal rpmint;
stream >> byte; // skip first byte
stream >> rpmint; // read float value
qDebug() << rpmint << "this is data to int" ;

back.setValue(rpmint);

KroMignon

@dziko147 said in Convert QbyteArray to qreal:

Currently I test a static QByteArray .
this is my code :
QByteArray rpmdata = "FF10001000100010001000100010001000";

What do you want to do with this?

If the data are hexadecimal coded, then you have to convert them to binary:

QByteArray rpmdata = QByteArray::fromHex("FF10001000100010001000100010001000");

==> https://doc.qt.io/qt-5/qbytearray.html#fromHex

dziko147

@KroMignon
So I try to receive data from QCanBusFrame .
So I extract the payload using : QByteArray QCanBusFrame::payload() const .
Currently I try to test a static QByteArray (I mean that i write manually the QByteArray) wich is the "rpmdata " .
the type of my value is real .
the frame compouned of 5byte( 1 for status and 4 for data) .
I hope that my problem looks clear .

Christian Ehrlicher

@dziko147 said in Convert QbyteArray to qreal:

I hope that my problem looks clear .

No since you still not tell us how the sender encodes the data and what value you expect.

KroMignon

@dziko147 said in Convert QbyteArray to qreal:

I hope that my problem looks clear .

Sorry but it is not clear to me :-(

What do represents "FF10001000100010001000100010001000", which is your test data?

dziko147

@KroMignon Exact . I write manually this test data .
@Christian-Ehrlicher I will receive a frame using QCanBusFrame , So the data will be in a QByteArray variable . I expect a Rpm Value wich is real .