Main Content

Pass Sparse Arrays to MATLAB from C++

MATLAB® sparse arrays provide efficient storage of double or logical data that has a large percentage of zeros. MATLAB sparse arrays support arithmetic, logical, and indexing operations. For more information, see Sparse Matrices.

This example shows how to use matlab::data::ArrayFactory to create a matlab::data::SparseArray array. It writes the data for the sparse array into buffers and uses these buffers to create the sparse array. Then the code passes the sparse array to MATLAB using MATLABEngine::setVariable.

#include "MatlabDataArray.hpp"
#include "MatlabEngine.hpp"

void sparseArray() {

    using namespace matlab::engine;

    // Connect to named shared MATLAB session started as:
    // matlab -r "matlab.engine.shareEngine('myMatlabEngine')"
    String session(u"myMatlabEngine");
    std::unique_ptr<MATLABEngine> matlabPtr = connectMATLAB(session);

    // Define the data for the sparse array
    std::vector<double> data = { 3.5, 12.98, 21.76 };
    std::vector<size_t> rows = { 0,0,1 };
    std::vector<size_t> cols = { 0, 4, 8 };
    size_t nnz = 3;

    // Create buffers for the data
    matlab::data::ArrayFactory factory;
    buffer_ptr_t<double> data_p = factory.createBuffer<double>(nnz);
    buffer_ptr_t<size_t> rows_p = factory.createBuffer<size_t>(nnz);
    buffer_ptr_t<size_t> cols_p = factory.createBuffer<size_t>(nnz);

    // Write data into the buffers
    double* dataPtr = data_p.get();
    size_t* rowsPtr = rows_p.get();
    size_t* colsPtr = cols_p.get();
    std::for_each(data.begin(), data.end(), [&](const double& e) { *(dataPtr++) = e; });
    std::for_each(rows.begin(), rows.end(), [&](const size_t& e) { *(rowsPtr++) = e; });
    std::for_each(cols.begin(), cols.end(), [&](const size_t& e) { *(colsPtr++) = e; });

    // Use the buffers to create the sparse array
    matlab::data::SparseArray<double> arr =
        factory.createSparseArray<double>({ 2,9 }, nnz, 
            std::move(data_p), std::move(rows_p), std::move(cols_p));

    // Put the sparse array in the MATLAB workspace
    matlabPtr->setVariable(u"s", arr);
}

The MATLAB whos command shows that the array passed to the MATLAB workspace is a sparse array.

>> whos
  Name      Size            Bytes  Class     Attributes

  s         2x9               128  double    sparse

For information on how to setup and build C++ engine programs, see Requirements to Build C++ Engine Applications.

See Also

| |