How to write entire QVector to a binary file?

[Christian Ehrlicher]

@CJha said in How to write entire QVector to a binary file?:

I have to read it in Matlab.

Then create your own format or check what matlab needs to read it with it's built-in functions.

[JonB]

@CJha
Then do not use a Qt serialization operator! Use whatever is going to be acceptable at the MatLab side, e.g. just a plain array's-worth of the data, or whatever. Qt serialization is for Qt programs!

[J.Hilk]

@CJha you can take a look into the source code

no idea what version you're using, but I found this

link

[Christian Ehrlicher]

@J-Hilk But this is internal and may change (although the chance is very very low)

[CJha]

@J-Hilk Thanks, I will go through the document and see if it can be helpful to me.
@Christian-Ehrlicher @JonB The only other way is to iterate through a QVector<double>, and is a time-consuming process, especially with millions of data points, and that's why I am interested in doing it directly.

[Christian Ehrlicher]

@CJha said in How to write entire QVector to a binary file?:

and is a time-consuming process, especially with millions of data points, and that's why I am interested in doing it directly.

Did you look at the QDataStream implementation? It does exactly the same... so why should this be faster?

[VRonin]

@CJha said in How to write entire QVector to a binary file?:

I have to read it in Matlab.

If I remeber correctly Matlab reads csv (aka text) so QTextStream is what you want. something like:

QFile file("BinTest.bin");
file.open(QIODevice::WriteOnly | QIODevice::Text);
QTextStream out(&file);
QVector<double> vec;
for(int ii = 0; ii < 10; ++ii){
    vec << ii * 0.33;
}
for(auto&& val : vec)
out << val << ',';
file.close();

[CJha]

@Christian-Ehrlicher I didn't know that before I saw the source code for QDataStream. I assumed since for a QVector data points are in adjacent memory positions pushing an entire vector into a binary file must be faster than iterating over it.

[JonB]

@CJha said in How to write entire QVector to a binary file?:

The only other way is to iterate through a QVector<double>, and is a time-consuming process, especially with millions of data points

Yep. That's what has to be done, and it's what the << serializer does, as @J-Hilk showed you. The only other way would be if you can get the address of contiguous QVector memory and save from there, which I'm guessing can be done. However...

...If @VRonin's latest post is correct and you're supposed to produce text instead to export then you cannot help but do it one-by-one....

P.S.
BTW, you'd have to test, but my guess is that code to output data points one-by-one instead of in a contiguous clump is not what would be slow over 1,000,000 points --- rather, the size of the output written to file will be what is significant....

[Christian Ehrlicher]

You're correct with the adjacent memory but QVector can also hold objects so memcpy'ing it out will not work there. You can use memcpy if you want in your case but QDataStream is generic and has no optimizations for such things.

[CJha]

@VRonin Yes, but writing a .csv file takes much longer than writing a binary file (almost 10 times more for large data sets). I have tried and tested it. I am gathering data at a much faster rate, up to 1 million doubles per second and I have to write it to a file continuously for hours, and this file will be analysed in Matlab by researchers. If I write 1 million data points in a .csv file it takes around 4 seconds while doing the same in a binary file takes around 400 milliseconds.

[JonB]

@CJha
OK, so STOP and tell us exactly what format MatLab will accept, and take it from there...

[jsulm]

@CJha But does Matlab support any binary format? You have to write in a format supported by Matlab.

[J.Hilk]

@CJha said in How to write entire QVector to a binary file?:

I am gathering data at a much faster rate, up to 1 million doubles per second and I have to write it to a file continuously for hours

actually stop right here!

if this program is used more than once, you're going to destroy your HD/SSD very quickly!

I'm sure there's an other - in memory - way to hand over those data points

[CJha]

@JonB @jsulm Matlab supports the binary format. There are functions such as fopen, fread, fseek etc. to read and write binary files. I have read binary file written using QDataStream in Matlab using fread and so on

[jsulm]

@CJha said in How to write entire QVector to a binary file?:

Matlab supports the binary format

Do you have its specification?

[CJha]

@J-Hilk I am not sure what you mean by

if this program is used more than once, you're going to destroy your HD/SSD very quickly!

Given that 1 million doubles are 8 million bytes, I think modern processors and disk drives can handle such speed easily.

[JonB]

@CJha
You may (well) know more than I, but can MatLab read and process 8MB of new data per second, at the same time as something else is producing it? And, separately, do you really generate 1 million new data points per second?

Also, as @J-Hilk said, wouldn't sending a pipe stream (e.g. a socket?) be better than writing to file and reading back? Does Matlab accept incoming data elsewhere than in a file?

[CJha]

@jsulm It is highly compatible. Here are the links for fopen, fread, fseek. I all these I can specify the format, ByteOrder, size of data (such as int, double, etc), and quite a few other things.
I don't think Matlab is the restrictive thing here, I can read any type of binary file in Matlab as long as I know how it is written.

[JonB]

@CJha
The code to write a QVector to file in the way you want, as fast as possible in one blob not one-by-one, is given in e.g. https://www.qtcentre.org/threads/65713-Output-a-QVector-of-doubles-to-a-binary-file-(without-using-QDatastream)?p=289540#post289540 :

qint64 bytesWritten = file.write(reinterpret_cast<const char*>(vec.constData()), sizeof(double) * vec.size());

EDIT I think you will want reinterpret_cast<> rather than static_cast<> here as shown in that post, so I have altered the code line to use that.