Converting to negative int - bug

[ShamusVW]

Hello
I am trying to use QString::toInt and have run into an issue that negative numbers get returned as zero.
Googling, I found http://bugreports.qt.nokia.com/brows...3Aall-tabpanel.

Can anyone help me get round this issue?

I have in my code

QString fl;
fl = QString(m_DataYPoint.toHex()); //convert the data to string
intValue = fl.toInt(0,16);  //...and the string to an int
m_YPoint = *(float*)&intValue;

QString fl;
fl = QString(m_DataYPoint.toHex()); //convert the data to string
intValue = fl.toInt(0,16);  //...and the string to an int
m_YPoint = *(float*)&intValue;

To copy to clipboard, switch view to plain text mode 

where
m_DataYPoint is of type QByteArray
intValue of type qint32
m_YPoint of type float.

Thanks,
Shaun

[wysota]

So where is the negative value and why are you converting an integer to a float in such a weird and incorrect way?

[ShamusVW]

I am reading data from a socket stored in m_DataYPoint (QByteArray).
I want to convert it to floating point, and until I realised that I also have negative possibilities, it worked fine.

As an example, m_DataYPoint might contain "BDC6D604". This should equal "-0.09708..." but it gets returned as 0.

...what is the correct way...?

[wysota]

As an example, m_DataYPoint might contain "BDC6D604". This should equal "-0.09708..." but it gets returned as 0.

If BDC6D604 is not an integer then why are you converting it to an integer?

...what is the correct way...?

A correct way to cast an int to float is:

int a = 7;
float b = a;

int a = 7;
float b = a;

To copy to clipboard, switch view to plain text mode 

Using pointers will cast pointers and make the internal representation of the integer be interpreted as an internal representation of a floating point value.

Anywho... If you read a float from the socket then first you have to ask the person who sent it what kind of encoding it uses (different platforms may have different encodings of floats or ints). Assuming it is the same as on your destination platform, simply do:

QByteArray ba("\xBD\xC6\xD6\x04",4);
float value;
memcpy(&value, ba.constData(), sizeof(float));

QByteArray ba("\xBD\xC6\xD6\x04",4);
float value;
memcpy(&value, ba.constData(), sizeof(float));

To copy to clipboard, switch view to plain text mode 

By the way, 0xBDC6D604 is 3,183,924,740 decimal (won't fit into 32 bit signed integer, so no way you would convert it through an int typed variable).

[ShamusVW]

I'm using IEEE-754 floating point.
Thanks, I will test it out.
Thanks for your input.

See http://babbage.cs.qc.edu/IEEE-754/32bit.html for correct conversion of floating point.

[tbscope]

That is something completely different than a int.

Look at documentation of IEEE-754 and the source code of that website to see how to convert it correctly.

toInt() certainly is not correct.

[wysota]

Just don't do the conversion (if you need any conversion at all!) by going through strings. If IEEE-754 is the officially used floating point representation then the code I gave you should work.

[ShamusVW]

Using pointers will cast pointers and make the internal representation of the integer be interpreted as an internal representation of a floating point value.

True - this is what I'm wanting, just not sure why negative is not working.
I have searched for ages looking for an efficient way to convert hex to floating point. When I originally implemented this solution, I'm not sure how I came upon it, but it worked. Unfortunately I was then only working in positive values.

Look at documentation of IEEE-754 and the source code of that website to see how to convert it correctly.

Ok, found the source! There seriously is not a shorter way to convert hex to float?

QByteArray ba("\xBD\xC6\xD6\x04",4);
float value;
memcpy(&value, ba.constData(), sizeof(float));

QByteArray ba("\xBD\xC6\xD6\x04",4);
float value;
memcpy(&value, ba.constData(), sizeof(float));

To copy to clipboard, switch view to plain text mode 

returns 0 again!

[wysota]

What is the contents of your byte array? Dump it to qDebug() please.

[ShamusVW]

For a background of my application, see http://www.qtcentre.org/threads/2509...ht=#post121381

I have a function

void Client::readSocket()
 {
    QByteArray block;
    block = tcpSocket->readAll();
    m_DataEccentricity[0] = block[9];
    m_DataEccentricity[1] = block[10];
    m_DataEccentricity[2] = block[11];
    m_DataEccentricity[3] = block[12];
    convertToFloat();
    ...
}

void Client::readSocket()
 {
    QByteArray block;
    block = tcpSocket->readAll();
    m_DataEccentricity[0] = block[9];
    m_DataEccentricity[1] = block[10];
    m_DataEccentricity[2] = block[11];
    m_DataEccentricity[3] = block[12];
    convertToFloat();
    ...
}

To copy to clipboard, switch view to plain text mode 

where

QByteArray m_DataEccentricity;

QByteArray m_DataEccentricity;

To copy to clipboard, switch view to plain text mode 

Convert to float...

void Client::convertToFloat()
{
    QString fl;
    fl = QString(m_DataEccentricity.toHex()); //convert the data to string
    qint32 intValue = fl.toInt(0,16);  //...and the string to an int
    m_Eccentricity = *(float*)&intValue;
    ...
}

void Client::convertToFloat()
{
    QString fl;
    fl = QString(m_DataEccentricity.toHex()); //convert the data to string
    qint32 intValue = fl.toInt(0,16);  //...and the string to an int
    m_Eccentricity = *(float*)&intValue;
    ...
}

To copy to clipboard, switch view to plain text mode 

and in another function where I do updating of the GUI

eccentricityLineEdit->setText(QString::number(m_Eccentricity,'f',6));

eccentricityLineEdit->setText(QString::number(m_Eccentricity,'f',6));

To copy to clipboard, switch view to plain text mode 

With regard to contents of my byte array, doing a

qDebug (DataEccentricity)

qDebug (DataEccentricity)

To copy to clipboard, switch view to plain text mode 

gives "¾ _h", but that isn't a blank between 3/4 and _, it is supposed to be a vertical bar with a small bar midway up pointing to the right (like an "H" without the right bar)

[wysota]

And using my code on it gives you 0?

[ShamusVW]

Yes.
Even your example you suggested in your post returns 0

QByteArray ba("\xBD\xC6\xD6\x04",4);
float value;
memcpy(&value, ba.constData(), sizeof(float));

QByteArray ba("\xBD\xC6\xD6\x04",4);
float value;
memcpy(&value, ba.constData(), sizeof(float));

To copy to clipboard, switch view to plain text mode 

...but I would like result of -0.09708788990974426 (as per IEEE-754 spec)