Matrix column extraction not working when using codegen

I'm trying to integrate some Matlab code with some C code, using codegen. For this task I need to extract some columns of data from a Matlab matrix.

The Matlab code runs fine but the C code does not appear to be extracting the columns correctly.

For example, if I have:

a=[1 2 3; 4 5 6; 7 8 9];

and I perform the following:

a1 = a(:,1);
a2 = a(:,2);
disp(a1);
disp(a2);

Then I get the expected output:

    1
    4
    7

    2
    5
    8

However, if I run codegen (on a slightly larger set) the C code is:

    memcpy(&a1[0], &a[0], 30U * sizeof(double));
    memcpy(&a2[0], &a[30], 30U * sizeof(double));

which results in the ouput:

    1
    2
    3
    
    4
    5
    6

Note: My actual data is read in from a large .csv file so I have simplified the details above to show the problem.

Also, with the large data files, the generated C code is using for loops rather than memcpy but that's by the by because the results are still the same.

Any suggestions would be most welcome.

Thanks very much,

John

Answer 1

[Mark McBroom]

By default, code from MATLAB coder is column major Try changing to row-major as described here:

https://www.mathworks.com/help/coder/ug/generate-code-that-uses-row-major-data.html#:~:text=To%20specify%20row-major%20layout%20with%20the%20MATLAB%20Coder,On%20the%20Memory%20tab%2C%20set%20Array%20layout%3A%20Row-major.

[John Edwards]

Hi @Mark McBroom,

Thank you very much for the quick response.

That worked great.

Best regards,

John