QByteArray to float

valerio.j

Hello,

I have an application going with a micro-controller sending voltage values as a string to be read by the Qt App (QtSerialPort). The string gets to Qt correctly, but now I have to convert this to a floating point so that I can do math operations. for example, when readADC(); is called I get a string with 24.21. How can I convert this to a floating int?

void MainWindow::readADC()
{

    QByteArray data;
        data = serial->readAll();
        qDebug() << data;

}

? Offline

Hi!

const QByteArray data = ...
bool ok = false;
const float = data.toFloat(&ok);
if (!ok) qDebug() << "Conversion failed";

See also: QByteArray

valerio.j

This is how I got it going, not sure if it's the best way of doing it, any input will be appreciated.

int MainWindow::readADC(int range)
{
    QByteArray data;
    float x, i, z, k, r;
    if (range == 24)
    {
        data = serial->readAll();
        qDebug() << data;
        x = static_cast<quint8>((data[0] -48) * 10);
        i = static_cast<quint8>((data[1] -48) * 1);
        z = static_cast<quint8>((data[3] -48) *.1);
        k = static_cast<quint8>((data[4] -48) * .01);
        r = x+i+z+k;
        //qDebug() << r;
        if ((r >= 23.3) && (r <= 24.7))
        return 1;
    }
    return 0;
}

kshegunov

@valerio.j said:

This is how I got it going, not sure if it's the best way of doing it, any input will be appreciated.

It's not. See what @Wieland wrote.

kuzulis

QDataStream in(serial, QIODevice::ReadOnly);
float value;
in >> value;

kshegunov

@kuzulis
Is this blocking (I'm curious for myself)?

PS.
Btw, he's sending it as a string. :)

kuzulis

It is just a simple example.. But real example it is something:

void Foo::onReadyRead()
{
    while (serial->bytesAvailable() >= 4) { // or, maybe QSataStream::atEnd()
        QDataStream in(serial);
        float value;
        in >> value;
    }
}

But, anyway, need to use serial->bytesAvailable() to control a number of received bytes, before reading.

Is this blocking (I'm curious for myself)?

No, why should it be blocking?

PS: It is just an additional way to @Wieland 's example.

Btw, he's sending it as a string. :)

Ahh, ok. :)

kshegunov

@kuzulis

No, why should it be blocking?

I'm not saying it should be; I was simply asking if it waits for the 4 bytes, or it tries to read them directly from the buffer. Non-blocking is just fine with me. :)

kuzulis

Btw, he's sending it as a string. :)

Then he need to add a '\n' after each string value, to allow to parse it correctly. Then need to use @Wieland 's solution. Or, maybe QTextStream.

valerio.j

Wielands solution worked for me.

Now I have another question.

Sending data from the Qt App. This results in an error, it only works if chkTstr24VDC was a QBytearray which I can deal with, but I'm trying to understand why it won't work with QString. Below is the error. Maybe I need to convert QString to QBytearray??

error: no matching function for call to 'QSerialPort::write(QString&)'
serial->write(chkTstr24VDC);
^

void MainWindow::on_powerON_clicked()
{
     int res;

    QString chkTstr24VDC ("$AR!ARP022#");

     serial->write(chkTstr24VDC);
     res = readADC(24);                                  //Read and test 24VDC range
}

mrjj

Hi
the write expects a const char *
To get that from Q String you can do

serial->write(text.toStdString().c_str());

http://doc.qt.io/qt-5/qiodevice.html#write
or use bytearray/ convert to.

kshegunov

@valerio-j

Maybe I need to convert QString to QBytearray??

Yes, indeed you need to do that. Also you need to be sure the port is ready after writing/before reading.

QByteArray chkTstr24VDC = QByteArrayLiteral("$AR!ARP022#");

serial->write(chkTstr24VDC);
serial->waitForBytesWritten(1000); //< Wait up to a second so the data is written to the port

serial->waitForReadyRead(1000);  //< Wait up to a second so the port is ready for reading
res = readADC(24);                                  // Read and test 24VDC range

@mrjj said:

To get that from Q String you can do
serial->write(text.toStdString().c_str());

But you shouldn't, you should rather enforce the encoding with toLatin1, toUtf8 or alike, and get a byte array from there. Then this byte array can be sent through the serial port. For example:

QString myStringData("some string data");
QByteArray byteArrayData = myStringData.toLatin1();

serial->write(byteArrayData);

valerio.j

@kshegunov said:

serial->waitForReadyRead(1000);

Thanks, this really improved my program, but it doesn't work 100% yet. Sometimes it combines both reads into a single string when using back to back write/reads. Below is what my code looks like now. I've tried with longer delays, but it doesn't help.

void MainWindow::on_powerON_clicked()
{
    int res24, res3v3;
    QPixmap stat_GO (":/res/images/stat_GO.png");
    QPixmap stat_noGO (":/res/images/stat_noGO.png");

    QByteArray chkTstr24VDC ("$AR!ARP022#");
    QByteArray chkBUT3_3VDC ("$AR^ARP032#");

    QByteArray powerON ("$DW!SHP079#");
    QByteArray kill ("$DW!SHP004#");
    QByteArray pwr_switch ("$DW!SLP058#");

    sndTesterData(powerON);                     //Enable 24VDC
    sndTesterData(chkTstr24VDC);                //Request 24VDC status to Tester

    serial->waitForReadyRead(1000);
    res24 = readADC(24);

    if(res24 == 1)
    {
    sndTesterData(kill);                        //E-OFF
    sndTesterData(pwr_switch);                  //On/Off Switch
    ui->_tstr24Vok->setPixmap(stat_GO);

        sndTesterData(chkBUT3_3VDC);
        serial->waitForReadyRead(1000);
        res3v3 = readADC(3);

        if(res3v3 == 1)
            ui->_but3v3Vok->setPixmap(stat_GO);
                else
                ui->_but3v3Vok->setPixmap(stat_noGO);
    }
    else
    ui->_tstr24Vok->setPixmap(stat_noGO);

}

kshegunov

@valerio.j said:

Thanks, this really improved my program, but it doesn't work 100% yet. Sometimes it combines both reads into a single string when using back to back write/reads.

This is not unexpected. There's buffering involved and there's no guarantee (as with TCP) that anything you write will be read as a separate packet. You need to ensure that the way you're sending the data is taking this into account (usually by using a protocol).