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Convert QbyteArray to qreal

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  • D dziko147

    @Christian-Ehrlicher I do not know how the sender encodes the data .
    What I know is I receive data using the QCanBusFrame . then I extract the data .
    So i get a QByteArray Data . I want to convert it .

    jsulmJ Offline
    jsulmJ Offline
    jsulm
    Lifetime Qt Champion
    wrote on last edited by
    #28

    @dziko147 said in Convert QbyteArray to qreal:

    I do not know how the sender encodes the data

    Then how do you want to decode it?
    You should at least get some data from sender where you know what exact numbers the sender sent, then you can try to find out how the data was encoded.

    https://forum.qt.io/topic/113070/qt-code-of-conduct

    1 Reply Last reply
    0
    • D dziko147

      @KroMignon
      I get value =0 after converting :/

      JonBJ Online
      JonBJ Online
      JonB
      wrote on last edited by JonB
      #29

      @dziko147
      After converting.... what? how?

      Tell you what: tell us two things:

      1. Forget about converting anything to float. Show us what the values of the first 5 bytes you receive are. (You say the first byte is a status byte and the next 4 bytes are the value you are interested in, right?)
      2. Tell us what floating point number you know that encodes/conveys/sends. (Please make sure it is not 0!)
      1 Reply Last reply
      1
      • D dziko147

        @KroMignon
        I get value =0 after converting :/

        KroMignonK Offline
        KroMignonK Offline
        KroMignon
        wrote on last edited by
        #30

        @dziko147 said in Convert QbyteArray to qreal:

        I get value =0 after converting :/

        After converting what?
        Please, take time to understand what are the data you want to convert.
        I cannot do this for you!

        It is an old maxim of mine that when you have excluded the impossible, whatever remains, however improbable, must be the truth. (Sherlock Holmes)

        D 1 Reply Last reply
        1
        • KroMignonK KroMignon

          @dziko147 said in Convert QbyteArray to qreal:

          I get value =0 after converting :/

          After converting what?
          Please, take time to understand what are the data you want to convert.
          I cannot do this for you!

          D Offline
          D Offline
          dziko147
          wrote on last edited by
          #31

          @KroMignon
          QByteArray rpmdata = QByteArray::fromHex("0102030405"); ---> I want to
          convert this .
          qDebug() << rpmdata << "this is data" ;
          QDataStream stream(rpmdata);
          quint8 byte;
          float rpmfloat;
          stream >> byte; // skip first byte
          stream >> rpmfloat; // read float value
          qDebug() << rpmfloat << "this is data to float" ;
          back.setValue(rpmfloat); ----> I display the value converted here .

          JonBJ KroMignonK 2 Replies Last reply
          0
          • D dziko147

            @KroMignon
            QByteArray rpmdata = QByteArray::fromHex("0102030405"); ---> I want to
            convert this .
            qDebug() << rpmdata << "this is data" ;
            QDataStream stream(rpmdata);
            quint8 byte;
            float rpmfloat;
            stream >> byte; // skip first byte
            stream >> rpmfloat; // read float value
            qDebug() << rpmfloat << "this is data to float" ;
            back.setValue(rpmfloat); ----> I display the value converted here .

            JonBJ Online
            JonBJ Online
            JonB
            wrote on last edited by JonB
            #32

            @dziko147
            You push raw bytes into a QDataStream. You then try to read that back as, say, a float.

            Unless I have misunderstood QDataStream you cannot do that. It is a structured stream. E.g. it will (well, should) contain "markers" for the data and its types.

            Experts, I am right here, aren't I (have never used QDataStream)?

            UPDATE
            For anyone reading this, the expert answers below indicate this is not the case, and my understanding was incorrect.

            Christian EhrlicherC KroMignonK 2 Replies Last reply
            0
            • JonBJ JonB

              @dziko147
              You push raw bytes into a QDataStream. You then try to read that back as, say, a float.

              Unless I have misunderstood QDataStream you cannot do that. It is a structured stream. E.g. it will (well, should) contain "markers" for the data and its types.

              Experts, I am right here, aren't I (have never used QDataStream)?

              UPDATE
              For anyone reading this, the expert answers below indicate this is not the case, and my understanding was incorrect.

              Christian EhrlicherC Offline
              Christian EhrlicherC Offline
              Christian Ehrlicher
              Lifetime Qt Champion
              wrote on last edited by
              #33

              @JonB said in Convert QbyteArray to qreal:

              E.g. it will (well, should) contain "markers" for the data and its types.

              Not for PODs. For QString e.g. a length field is added. What is added for which build-in Qt type is not documented - QDataStream is not meant to be used with anything else than a QDataStream.

              Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
              Visit the Qt Academy at https://academy.qt.io/catalog

              1 Reply Last reply
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              • JonBJ JonB

                @dziko147
                You push raw bytes into a QDataStream. You then try to read that back as, say, a float.

                Unless I have misunderstood QDataStream you cannot do that. It is a structured stream. E.g. it will (well, should) contain "markers" for the data and its types.

                Experts, I am right here, aren't I (have never used QDataStream)?

                UPDATE
                For anyone reading this, the expert answers below indicate this is not the case, and my understanding was incorrect.

                KroMignonK Offline
                KroMignonK Offline
                KroMignon
                wrote on last edited by
                #34

                @JonB said in Convert QbyteArray to qreal:

                Unless I have misunderstood QDataStream you cannot do that. It is a structured stream. E.g. it will (well, should) contain "markers" for the data and its types.

                Of course this is possible, this is no "marker" for serializing/deserializing "base types" like float, double, q(u)int8, q(u)int16, etc.

                But "02030405" converted to float is 9.6255135462533111609068148127E-38... almost 0 ;)

                It is an old maxim of mine that when you have excluded the impossible, whatever remains, however improbable, must be the truth. (Sherlock Holmes)

                1 Reply Last reply
                2
                • D dziko147

                  @KroMignon
                  QByteArray rpmdata = QByteArray::fromHex("0102030405"); ---> I want to
                  convert this .
                  qDebug() << rpmdata << "this is data" ;
                  QDataStream stream(rpmdata);
                  quint8 byte;
                  float rpmfloat;
                  stream >> byte; // skip first byte
                  stream >> rpmfloat; // read float value
                  qDebug() << rpmfloat << "this is data to float" ;
                  back.setValue(rpmfloat); ----> I display the value converted here .

                  KroMignonK Offline
                  KroMignonK Offline
                  KroMignon
                  wrote on last edited by
                  #35

                  @dziko147 said in Convert QbyteArray to qreal:

                  QByteArray rpmdata = QByteArray::fromHex("0102030405"); ---> I want to
                  convert this .

                  What did you expect?
                  Again, you have to know what you are doing.

                  Here you are converting this array of bytes { 0x02, 0x03, 0x04, 0x05 } to float.
                  which is 9.6255135462533111609068148127E-38, almost 0.

                  If you change the array to something else like "014F030405", which means converting { 0x4f, 0x03, 0x04 , 0x05} to float, you will obtain 2.19807872E9 (almost 2.2 Billion)

                  ==> cf. https://www.binaryconvert.com/convert_float.html

                  It is an old maxim of mine that when you have excluded the impossible, whatever remains, however improbable, must be the truth. (Sherlock Holmes)

                  D 1 Reply Last reply
                  2
                  • KroMignonK KroMignon

                    @dziko147 said in Convert QbyteArray to qreal:

                    QByteArray rpmdata = QByteArray::fromHex("0102030405"); ---> I want to
                    convert this .

                    What did you expect?
                    Again, you have to know what you are doing.

                    Here you are converting this array of bytes { 0x02, 0x03, 0x04, 0x05 } to float.
                    which is 9.6255135462533111609068148127E-38, almost 0.

                    If you change the array to something else like "014F030405", which means converting { 0x4f, 0x03, 0x04 , 0x05} to float, you will obtain 2.19807872E9 (almost 2.2 Billion)

                    ==> cf. https://www.binaryconvert.com/convert_float.html

                    D Offline
                    D Offline
                    dziko147
                    wrote on last edited by
                    #36

                    @KroMignon I tried this :
                    QByteArray rpmdata;
                    rpmdata.resize(4);
                    rpmdata[0] = 0x43;
                    rpmdata[1] = 0x96;
                    rpmdata[2] = 0x7D;
                    rpmdata[3] = 0xA6;
                    qDebug() << rpmdata << "this is data" ;

                    QDataStream stream(rpmdata);

                    float value;
                    stream >> value; // read float value
                    qDebug() << value << "this is data to float" ;

                    back.setValue(value);

                    I get :
                    deb.PNG

                    the 43967DA6 = (300.98163 ) .
                    this is the the variable rpm.PNG

                    KroMignonK 1 Reply Last reply
                    0
                    • D dziko147

                      @KroMignon I tried this :
                      QByteArray rpmdata;
                      rpmdata.resize(4);
                      rpmdata[0] = 0x43;
                      rpmdata[1] = 0x96;
                      rpmdata[2] = 0x7D;
                      rpmdata[3] = 0xA6;
                      qDebug() << rpmdata << "this is data" ;

                      QDataStream stream(rpmdata);

                      float value;
                      stream >> value; // read float value
                      qDebug() << value << "this is data to float" ;

                      back.setValue(value);

                      I get :
                      deb.PNG

                      the 43967DA6 = (300.98163 ) .
                      this is the the variable rpm.PNG

                      KroMignonK Offline
                      KroMignonK Offline
                      KroMignon
                      wrote on last edited by KroMignon
                      #37

                      @dziko147 said in Convert QbyteArray to qreal:

                      the 43967DA6 = (300.98163 ) .

                      Sorry, my bad, it seems there are something I had forgot about QDataStream: when using float you have to specify floating point precision with stream.setFloatingPointPrecision(QDataStream::SinglePrecision);.

                      The other way is to use qfloat16 (WRONG)

                      QByteArray rpmdata;
                      rpmdata.resize(4);
                      rpmdata[0] = 0x43;
                      rpmdata[1] = 0x96;
                      rpmdata[2] = 0x7D;
                      rpmdata[3] = 0xA6;
                      qDebug() << rpmdata << "this is data" ;
                      
                      QDataStream stream(rpmdata);
                      
                      stream.setFloatingPointPrecision(QDataStream::SinglePrecision);
                      float value;
                      stream >> value; // read float value
                      qDebug() << value << "this is data to float" ;
                      
                      back.setValue(value);
                      

                      It is an old maxim of mine that when you have excluded the impossible, whatever remains, however improbable, must be the truth. (Sherlock Holmes)

                      1 Reply Last reply
                      4
                      • D Offline
                        D Offline
                        dziko147
                        wrote on last edited by
                        #38

                        @KroMignon thank you it works :) .

                        1 Reply Last reply
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