How do I call Matlab polyfit function from C++?

Hello, I am trying to call the MATLAB polyfit function from a Windows application written in Visual Studio 2022 C++ and get the polynomial coefficients array, s, and mu from from polyfit.

int main(int argc, char* argv[])
{
    double x[40];                 
    double y[40];
    double p[12];  // MATLAB returned array of coefficients

    size_t dataSize;


    // this function gets the data from a file and updates the unknown xy data size 
    // the data size = 40 elements
    getXYpoints(x, y, &dataSize);

    if (dataSize == 0)
    {
        cout << endl << "File Size is Zero, nothing to do";
        return -2;
    }

    polyFit(x, y, p,dataSize, 12);  // 12 is the order of the polynomials
}

The polyFit calls the MATLAB polyfit function

void polyFit(double* x, double* y, double* poly_coefficients, size_t xy_size, size_t poly_size)
{
    using namespace matlab::engine;

    // Start MATLAB engine synchronously
    std::unique_ptr<MATLABEngine> matlabPtr = startMATLAB();

    std::tuple<double, double, double> nresults;
    
// the following line gives a compile error for x, y, and the tuple definitions
    nresults = matlabPtr->feval<std::tuple <double*, double, double>>(u"polyfit", x , y, poly_size);

    //[p, s, mu] = polyfit (x,y, 12);
    double P;
    double S;
    double MU;
    std::tie(P, S, MU) = nresults;


    for (auto it : p)
    {
        p[it] = P[it];
    }
    auto s = S;
    auto mu = MU;
    .
    .
    .
    
}

tuple <double*, double, double> tuple does not seem to support the first argument double*. Is there a better way to call polyfit from C++, or how do I overload the tuple class to accept a pointer to P (the polynomial coefficients array).

Answer 1

[Nidal Bazzi]

Thank you, the last update seems to work, there are no more exceptions when the code runs but I am unable to access the p, s, and mu in C++. p is supposed to be a 1X13 array of double, s is a 1X1 struct with fields R (13X13) double, df is integer, normr is a double, mu is 2X1 array of double.

    FutureResult<std::vector<Array>> result = matlabPtr->fevalAsync(u"polyfit", 3, { mxArrayX, mxArrayY, factory.createScalar<double>(poly_size) }); // line 64
    .
    .
    .
    // Extract s and mu if needed
    auto s_field1 = s[0][u"normr"];      // line 81
    double mu_field1 = mu[0][u"field1"]; // line 82
    std::cout >> poly_coefficients[0];   // line 83
    

These are the errors in Visual Studio

Rebuild started at 12:59 PM...

1>------ Rebuild All started: Project: ectCalFit, Configuration: Debug x64 ------

1>matlab_functions.cpp

1>D:\OneDrive - azureford\Documents\Visual Studio 2022\Projects\ectCalFit\ectCalFit\matlab_functions.cpp(64,135): warning C4244: 'argument': conversion from 'size_t' to 'const T', possible loss of data

1>D:\OneDrive - azureford\Documents\Visual Studio 2022\Projects\ectCalFit\ectCalFit\matlab_functions.cpp(64,135): warning C4244: with

1>D:\OneDrive - azureford\Documents\Visual Studio 2022\Projects\ectCalFit\ectCalFit\matlab_functions.cpp(64,135): warning C4244: [

1>D:\OneDrive - azureford\Documents\Visual Studio 2022\Projects\ectCalFit\ectCalFit\matlab_functions.cpp(64,135): warning C4244: T=double

1>D:\OneDrive - azureford\Documents\Visual Studio 2022\Projects\ectCalFit\ectCalFit\matlab_functions.cpp(64,135): warning C4244: ]

1>D:\OneDrive - azureford\Documents\Visual Studio 2022\Projects\ectCalFit\ectCalFit\matlab_functions.cpp(81,24): error C2679: binary '[': no operator found which takes a right-hand operand of type 'const char16_t [6]' (or there is no acceptable conversion)

1>C:\Program Files\MATLAB\R2020b\extern\include\MatlabDataArray\ArrayElementTypedRef.hpp(108,55):

1>could be 'matlab::data::ArrayElementTypedRef<T,false> matlab::data::ArrayElementTypedRef<matlab::data::Struct,false>::operator [](std::string)'

1> with

1> [

1> T=matlab::data::Array

1> ]

1>C:\Program Files\MATLAB\R2020b\extern\include\MatlabDataArray\ArrayElementTypedRef.hpp(86,51):

1>or 'matlab::data::ArrayElementTypedRef<matlab::data::Struct,false> matlab::data::ArrayElementTypedRef<matlab::data::Struct,false>::operator [](size_t)'

1>D:\OneDrive - azureford\Documents\Visual Studio 2022\Projects\ectCalFit\ectCalFit\matlab_functions.cpp(81,24):

1>while trying to match the argument list '(matlab::data::ArrayElementTypedRef<matlab::data::Struct,false>, const char16_t [6])'

1>D:\OneDrive - azureford\Documents\Visual Studio 2022\Projects\ectCalFit\ectCalFit\matlab_functions.cpp(82,28): error C2679: binary '[': no operator found which takes a right-hand operand of type 'const char16_t [7]' (or there is no acceptable conversion)

1>C:\Program Files\MATLAB\R2020b\extern\include\MatlabDataArray\ArrayElementTypedRef.hpp(108,55):

1>could be 'matlab::data::ArrayElementTypedRef<T,false> matlab::data::ArrayElementTypedRef<double,false>::operator [](std::string)'

1> with

1> [

1> T=matlab::data::Array

1> ]

1>C:\Program Files\MATLAB\R2020b\extern\include\MatlabDataArray\ArrayElementTypedRef.hpp(86,51):

1>or 'matlab::data::ArrayElementTypedRef<T,false> matlab::data::ArrayElementTypedRef<T,false>::operator [](size_t)'

1> with

1> [

1> T=double

1> ]

1>D:\OneDrive - azureford\Documents\Visual Studio 2022\Projects\ectCalFit\ectCalFit\matlab_functions.cpp(82,28):

1>while trying to match the argument list '(matlab::data::ArrayElementTypedRef<T,false>, const char16_t [7])'

1> with

1> [

1> T=double

1> ]

1>D:\OneDrive - azureford\Documents\Visual Studio 2022\Projects\ectCalFit\ectCalFit\matlab_functions.cpp(83,15): error C2676: binary '>>': 'std::ostream' does not define this operator or a conversion to a type acceptable to the predefined operator

1>C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\Tools\MSVC\14.38.33130\include\xstring(5144,32):

1>could be 'std::basic_istream<_Elem,_Traits> &std::operator >>(std::basic_istream<_Elem,_Traits> &,std::basic_string<_Elem,_Traits,_Alloc> &)'

1> D:\OneDrive - azureford\Documents\Visual Studio 2022\Projects\ectCalFit\ectCalFit\matlab_functions.cpp(83,10):

1> 'std::basic_istream<_Elem,_Traits> &std::operator >>(std::basic_istream<_Elem,_Traits> &,std::basic_string<_Elem,_Traits,_Alloc> &)': could not deduce template argument for 'std::basic_istream<_Elem,_Traits> &' from 'std::ostream'

1>C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\Tools\MSVC\14.38.33130\include\istream(842,32):

1>or 'std::basic_istream<_Elem,_Traits> &std::operator >>(std::basic_istream<_Elem,_Traits> &,_Elem *)'

1> D:\OneDrive - azureford\Documents\Visual Studio 2022\Projects\ectCalFit\ectCalFit\matlab_functions.cpp(83,10):

1> 'std::basic_istream<_Elem,_Traits> &std::operator >>(std::basic_istream<_Elem,_Traits> &,_Elem *)': could not deduce template argument for 'std::basic_istream<_Elem,_Traits> &' from 'std::ostream'

1>C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\Tools\MSVC\14.38.33130\include\istream(847,31):

1>or 'std::basic_istream<char,_Traits> &std::operator >>(std::basic_istream<char,_Traits> &,signed char *)'

1> D:\OneDrive - azureford\Documents\Visual Studio 2022\Projects\ectCalFit\ectCalFit\matlab_functions.cpp(83,10):

1> 'std::basic_istream<char,_Traits> &std::operator >>(std::basic_istream<char,_Traits> &,signed char *)': could not deduce template argument for 'std::basic_istream<char,_Traits> &' from 'std::ostream'

1>C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\Tools\MSVC\14.38.33130\include\istream(852,31):

1>or 'std::basic_istream<char,_Traits> &std::operator >>(std::basic_istream<char,_Traits> &,unsigned char *)'

1> D:\OneDrive - azureford\Documents\Visual Studio 2022\Projects\ectCalFit\ectCalFit\matlab_functions.cpp(83,10):

1> 'std::basic_istream<char,_Traits> &std::operator >>(std::basic_istream<char,_Traits> &,unsigned char *)': could not deduce template argument for 'std::basic_istream<char,_Traits> &' from 'std::ostream'

1>C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\Tools\MSVC\14.38.33130\include\istream(858,32):

1>or 'std::basic_istream<_Elem,_Traits> &std::operator >>(std::basic_istream<_Elem,_Traits> &,_Elem &)'

1> D:\OneDrive - azureford\Documents\Visual Studio 2022\Projects\ectCalFit\ectCalFit\matlab_functions.cpp(83,10):

1> 'std::basic_istream<_Elem,_Traits> &std::operator >>(std::basic_istream<_Elem,_Traits> &,_Elem &)': could not deduce template argument for 'std::basic_istream<_Elem,_Traits> &' from 'std::ostream'

1>C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\Tools\MSVC\14.38.33130\include\istream(881,31):

1>or 'std::basic_istream<char,_Traits> &std::operator >>(std::basic_istream<char,_Traits> &,signed char &)'

1> D:\OneDrive - azureford\Documents\Visual Studio 2022\Projects\ectCalFit\ectCalFit\matlab_functions.cpp(83,10):

1> 'std::basic_istream<char,_Traits> &std::operator >>(std::basic_istream<char,_Traits> &,signed char &)': could not deduce template argument for 'std::basic_istream<char,_Traits> &' from 'std::ostream'

1>C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\Tools\MSVC\14.38.33130\include\istream(887,31):

1>or 'std::basic_istream<char,_Traits> &std::operator >>(std::basic_istream<char,_Traits> &,unsigned char &)'

1> D:\OneDrive - azureford\Documents\Visual Studio 2022\Projects\ectCalFit\ectCalFit\matlab_functions.cpp(83,10):

1> 'std::basic_istream<char,_Traits> &std::operator >>(std::basic_istream<char,_Traits> &,unsigned char &)': could not deduce template argument for 'std::basic_istream<char,_Traits> &' from 'std::ostream'

1>C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\Tools\MSVC\14.38.33130\include\istream(900,9):

1>or '_Istr &&std::operator >>(_Istr &&,_Ty &&)'

1> D:\OneDrive - azureford\Documents\Visual Studio 2022\Projects\ectCalFit\ectCalFit\matlab_functions.cpp(83,10):

1> '_Istr &&std::operator >>(_Istr &&,_Ty &&)': could not deduce template argument for '__formal'

1> C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\Tools\MSVC\14.38.33130\include\istream(899,102):

1> '<template-parameter>': you cannot create a pointer to a reference

1>C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\Tools\MSVC\14.38.33130\include\complex(2235,28):

1>or 'std::basic_istream<_Elem,_Traits> &std::operator >>(std::basic_istream<_Elem,_Traits> &,std::complex<_Other> &)'

1> D:\OneDrive - azureford\Documents\Visual Studio 2022\Projects\ectCalFit\ectCalFit\matlab_functions.cpp(83,10):

1> 'std::basic_istream<_Elem,_Traits> &std::operator >>(std::basic_istream<_Elem,_Traits> &,std::complex<_Other> &)': could not deduce template argument for 'std::basic_istream<_Elem,_Traits> &' from 'std::ostream'

1>PolyFit.cpp

1>Generating Code...

1>Done building project "ectCalFit.vcxproj" -- FAILED.

========== Rebuild All: 0 succeeded, 1 failed, 0 skipped ==========

========== Rebuild completed at 12:59 PM and took 04.647 seconds ==========

Answer 2

[Hassaan]

#include <iostream>
#include "MatlabEngine.hpp"
#include "MatlabDataArray.hpp"

void getXYpoints(double* x, double* y, size_t* dataSize);

void polyFit(double* x, double* y, double* poly_coefficients, size_t xy_size, size_t poly_size) {
    using namespace matlab::engine;
    using namespace matlab::data;

    // Start MATLAB engine synchronously
    std::unique_ptr<MATLABEngine> matlabPtr = startMATLAB();

    // Create MATLAB data arrays
    ArrayFactory factory;
    TypedArray<double> mxArrayX = factory.createArray<double>({xy_size});
    TypedArray<double> mxArrayY = factory.createArray<double>({xy_size});

    for (size_t i = 0; i < xy_size; ++i) {
        mxArrayX[i] = x[i];
        mxArrayY[i] = y[i];
    }

    // Call polyfit and get the result
    FutureResult<std::vector<Array>> result = matlabPtr->fevalAsync(u"polyfit", 3, {mxArrayX, mxArrayY, factory.createScalar<double>(poly_size)});
    std::vector<Array> results = result.get();

    TypedArray<double> poly_coefficients_array = results[0];
    StructArray s = results[1];
    TypedArray<double> mu = results[2];

    // Extract polynomial coefficients
    for (size_t i = 0; i < poly_coefficients_array.getNumberOfElements(); ++i) {
        poly_coefficients[i] = poly_coefficients_array[i];
    }

    // Extract s and mu if needed
    // auto s_field1 = s[0][u"field1"];
    // auto mu_field1 = mu[0][u"field1"];
}

int main(int argc, char* argv[]) {
    double x[40];
    double y[40];
    double p[12];  // MATLAB returned array of coefficients
    size_t dataSize;
    // This function gets the data from a file and updates the unknown xy data size
    // The data size = 40 elements
    getXYpoints(x, y, &dataSize);
    if (dataSize == 0) {
        std::cout << "File Size is Zero, nothing to do" << std::endl;
        return -2;
    }
    polyFit(x, y, p, dataSize, 12);  // 12 is the order of the polynomials
    return 0;
}

------------------------------------------------------------------------------------------------------------------------------------------------

If you find the solution helpful and it resolves your issue, it would be greatly appreciated if you could accept the answer. Also, leaving an upvote and a comment are also wonderful ways to provide feedback.

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Answer 3

[Nidal Bazzi]

Thanks Hassaan for the quick response. Line 56 gives these errors.

error C2440: 'initializing': cannot convert from 'matlab::data::Array' to 'std::vector<matlab::data::Array,std::allocator<matlab::data::Array>>'

line 56:

std::vector<Array> results = matlabPtr->feval(u"polyfit", { mxArrayX, mxArrayY, factory.createScalar<double>(poly_size) });

[Hassaan]

@Nidal Bazzi I have updated my code above. Try re-run.

Answer 4

[Nidal Bazzi]

It still will not compile unless I added the 3 on this line:

FutureResult<std::vector<Array>> result = matlabPtr->fevalAsync(u"polyfit",3, { mxArrayX, mxArrayY, factory.createScalar<double>(poly_size) });

polyfit defaults to returning one argument unless we specify 3 argument. This change enable the code to compile, but an exception is encountered on this line:

StructArray mu = results[2];

[Hassaan]

@Nidal Bazzi I have updated my code above. Try re-run.

Answer 5

[Nidal Bazzi]

It turned out that I only need the polynomial coefficients vector p which I am able to get. Thank you for all your help.

[Nidal Bazzi]

@Hassaan the last update fixed my issue.

[Hassaan]

@Nidal Bazzi You are welcome.

------------------------------------------------------------------------------------------------------------------------------------------------

If you find the solution helpful and it resolves your issue, it would be greatly appreciated if you could accept the answer. Also, leaving an upvote and a comment are also wonderful ways to provide feedback.

Professional Interests

• Technical Services and Consulting
• Embedded Systems | Firmware Developement | Simulations
• Electrical and Electronics Engineering

Feel free to contact me.