Converting coder::array<unsigned char, 3U> to unsigned char

I am using Matlab coder to generate c++ codes for a script i have made. The output datatype of MATLAB coder is coder::array<unsigned char, 3U>. Would you please help me figure out how to convert this into unsigned char so that i can use it in opencv.

Thank you

[Alexander Bottema]

To directly access the underying data of a coder::array you can use the ".data()" method. The total number of elements can be computed via the ".numel()" method. Size of each dimension is available via the ".size(dimension)" method.

I attached a full example (see example.zip).

example.m:

function y = example(n)

y = zeros(n,n,n,'uint8');

for i = 1:numel(y)

y(i) = 64 + i;

end

main.cpp:

#include <iostream>
#include "example.h"
#include "coder_array.h"

int main(int argc, char **argv[])
{
    (void)argc;
    (void)argv;

    coder::array<unsigned char, 3U> out;
    example(2, out);

    int n = out.numel();
    unsigned char *data = out.data();
    for (int i = 0; i < n; i++) {
        std::cout << data[i];
    }
    std::cout << std::endl;

    return 0;
}

doit.m:

cfg = coder.config('exe');

cfg.TargetLang = 'C++';

codegen -args 0 -config cfg example main.cpp

And running the executable gives:

>> !example (!./example on Linux)

ABCDEFGH

Answer 1

[Anirban Ghosh]

Hi Abhijith,

Please refer to this documentation page for information on using the coder:array template: Use Dynamically Allocated C++ Arrays in Generated Function Interfaces. In particular, note the "Using the coder::array Class Template" section and the two examples that follow.

If your output (say myArray) is of type coder::array<unsigned char, 3U>, it contains elements of type unsigned char and has 3 dimensions.

• To access the sizes of the three dimensions, call the size method: myArray.size(0), myArray.size(1), and myArray.size(2).
• To access the (i,j,k)-th element of myArray, call the at method: myArray.at(i,j,k). This call will return the unsigned char value stored at that location.
• In your code, you can pre-allocate an unsigned char array and populate it with these values.

[xingxingcui]

hi, @Anirban Ghosh，@Alexander Bottema

How should I set the data elements of coder::array if the other way around?（Instead of copying element by element, the in-place method）, I'm following the linked documentation and trying myself, but unfortunately I couldn't make it work!

https://www.mathworks.com/matlabcentral/answers/1916640-matlab-coder-how-should-the-coder-array-member-function-set-convert-opencv-s-mat-type-to-its-type