QT - Writing data input to csv with QTextStream

Question

I have data coming in from an Arduino, which I want to write to a CSV file. The problem is the data keeps overwriting the existing line. Here's a sample of the code:

QFile data("F:/logdata.csv");


if (data.open(QFile::WriteOnly | QFile::Truncate)) {
 QTextStream out(&data);



 out << hum << "," << temp << "," << gas << '\n';


 }

Answer 1

Are you trying to reopen and append data to the same file? In that case you don't want to use the QFile::Truncate flag since it will remove the content in that file.

Please see http://doc.qt.io/qt-5/qiodevice.html :

QIODevice::Truncate If possible, the device is truncated before it is opened. All earlier contents of the device are lost.

Also as suggested by Ibarros use the QIODevice::Append flag to make sure data is appended.

Update

Actually you also don't want to use the QIODevice::WriteOnly flag too, since it will add the truncate flag. You should use QIODevice::ReadWrite instead.

data.open(QIODevice::ReadWrite| QIODevice::Append)

Answer 2

When opening the file, use "QIODevice::Append" flag, so the data you write to it will be appended at the end of the file.

Example:

data.open(QFile::WriteOnly | QFile::Truncate |QIODevice::Append)