Linear indexing for multi-dimensional struct array in C++ API

In plain MATLAB I can use the following code and ignore the actual dimensions:

s = repmat(struct('exampleField',1),[2,3,4,5]);
for ii = 1:numel(s)
      disp(s(ii).exampleField)
end

However, in the C++ API, I can't use the linear indexing as the following code fails with the error: "Not enough indices provided."

void func(matlab::data::StructArray s){
for (size_t ii = 0; ii < s.getNumberOfElements(); ++ii)
      std::cout << s[ii][std::string("exampleField")] << std::endl;
}

Is there a way to still use linear indexing? Without linear indexing generic code development gets nearly impossible.

Answer 1

[MathWorks Support Team]

Linear indexing can be achieved using the 'matlab::data::TypedIterator<T>'. This is a templated C++ class which provides random access iterator in memory order and can be used to iterate and print the struct array. Further details about this class can be found in the link below. 

https://www.mathworks.com/help/releases/R2021a/matlab/apiref/matlab.data.typediterator.html?searchHighlight=matlab.data.typediterator&s_tid=doc_srchtitle

The following code describes two ways to assign and display the values in a struct array.

#include "MatlabDataArray.hpp" 
#include "mex.hpp" 
#include "mexAdapter.hpp" 

using namespace matlab::data; 
using matlab::mex::ArgumentList; 

class MexFunction : public matlab::mex::Function { 
    public: 
    void operator()(ArgumentList outputs, ArgumentList inputs)  
    { 
        ArrayFactory factory; 
        StructArray S = factory.createStructArray({2, 3, 4, 5 }, { "exampleField" }); 

        // Assign and display using Iterator 
        TypedIterator<Struct> it(S.begin()); 
        for (size_t i=0; i<S.getNumberOfElements(); i++) 
        { 
            // Assign values to exampleField 
            it[i]["exampleField"] = factory.createArray<int>({1, 1}, {(int)i}); 

            // Display value 
            std::cout << "S[" << i << "] = " << (int)(it[i]["exampleField"])[0] << std::endl; 
        } 

        // Another way to assign and display values 
        int cnt = 0; 
        for (auto& elem : S) { 
            elem["exampleField"] = factory.createArray<int>({1, 1}, {2*cnt}); 
            cnt = cnt + 1; 
            std::cout << "S[" << cnt << "] = " << (int)(elem["exampleField"])[0] << std::endl;        
        } 
    } 
};