Convert QbyteArray to qreal

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 .

Christian Ehrlicher

@dziko147 said in Convert QbyteArray to qreal:

. I expect a Rpm Value wich is real .

This tells us nothing. real just tells that the value is a floating-point value. It does not tell how this value is encoded and since you can't/won't tell us this you have to find it out by yourself. Two possibilities were already given to you, other encodings are unknown to us and therefore you're on your own when none of them works.