使用数据接口将 C++ 数据存储到八度音阶？

Question

Matlab offers a data interface for C/C++, see here . This allows users to export data from C/C++ into at runtime into Matlab formats (*.mat files). This means that you can export data from a C++ program and later return to the data using Matlab to analyze it.

I would like to start using an open-source alternative, Octave, in stead of Matlab. However, I am confused by the C/C++ interfaces that octave offers. What I understand so far is that I can call external routines from Octave using their external code interface .

But can I also do the reverse, and store data from my C++ program to octave during runtime? If yes, pointers to a minimal example would be highly appreciated.

Answer 1

This is probably not an answer for you, but writing it as a comment was too long and dense, so I'm writing as an answer for convenience, plus hopefully while trying to clarify things it may provide you with an answer anyway.

Essentially, there is some confusion in your question. It's worth clarifying these points first. If my comments here don't answer your question, perhaps you can edit your question to clarify what you want exactly based on my comments below.

1. Matlab offers two interfaces, one for C and one for C++. These are completely separate interfaces.

2. These are general interfaces, for writing a function in, say, C, and calling that function from matlab, or vice versa. They are not for "exporting data". It may be possible to store data from within C code into a.mat by calling the matlab "save" function via the provided api, but I can't think of a good reason to do this via C rather than as part of a matlab script which calls a C-written function to process some data, and then calls 'save' from matlab to save the result.

3. Octave supports matlab's "C" interface (so called ".mex" files). It does not support matlab's "C++" interface. But it does have its own "C++" interface, namely ".oct" files.

4. If you have an example of a.mex file which performs what you ask, then presumably it can also be compiled in octave with little to no modification (except for including the right library names to reflect octave headers).

5. "I can call external routines from Octave" <-- you can assign general C++ code a function name and transform it, so that you can call it from octave. More to the point though, you can pass this function inputs from octave, let it handle the computations in C++, and then return a result back to octave. Is this what you mean by this? If so, then yes, this is possible, and the manual section you linked has examples of how to do this (btw, note that you are linking to an older version, the newest version of the manual is octave.org/doc/v5.2.0 )

6. Octave also provides an interface for calling Octave functions directly from C++ code (whether in the context of.oct files intended for use within octave, or in the context of standalone C++ executables which would like to use octave functions inside).

When you say "store data", it seems to me that you are basically asking about point No.5, ie pass data as an input to a c++ written function, have c++ do something with that data, and then return its result back to the octave workspace. Is that correct? Or are you literally asking for a way to create a.mat file from within c++ specifically?

If what you're asking for is simply the former, then the manual provides many examples. Eg if you wanted to try the one near the end of this page: https://octave.org/doc/v5.2.0/Matrices-and-Arrays-in-Oct_002dFiles.html#Matrices-and-Arrays-in-Oct_002dFiles , then say that code is saved in file addtwomatrices.cpp , then you can compile (eg from your linux terminal) it like so:

$ mkoctfile -c addtwomatrices.cpp
$ mkoctfile ./addtwomatrices.o -o ./addtwomatrices

And then call it from octave like so:

> addtwomatrices( eye(2), magic(2) )
ans =
   5   3
   1   3

UPDATE

Here is an example of creating a matrix directly from C++ and then saving it to an octave-compatible format.

// in testmat.cpp
# include <octave/oct.h>
# include <octave/ls-oct-text.h>   /* defines save_text_data */

int main ()
{
 // Create a matrix and fill it with values
    Matrix M(3,3);
    for (int i = 0;  i < 9;  ++i )   { M.elem(i) = i+1.; }

 // Save to disk
    std::ofstream out_file_stream( "out.mat" );
    save_text_data ( out_file_stream,
                     octave_value(M),
                     std::string("M_matrix"),   // variable name
                     true,                      // mark as global?
                     5                          // precision
                   );

    out_file_stream.close();
}

Compile, link, and run:

mkoctfile -c testmat.cpp
mkoctfile --link-stand-alone -o testmat testmat.o
LD_LIBRARY_PATH=/opt/octave-5.2.0/lib/octave/5.2.0 ./testmat   # change path accordingly

Confirm from octave out.mat was saved properly and can be opened in octave:

octave:1> load out.mat
octave:2> M_matrix
M_matrix =
   1   4   7
   2   5   8
   3   6   9