Work with MATLAB Structure Arrays - MATLAB & Simulink (original) (raw)

In MATLAB®, you can create struct arrays by dynamically assigning field names and values. In contrast, C++ is a statically typed language that requires all fields of a struct and their types to be declared upfront. This discussion uses the exampleDeploy MATLAB Function That Accepts Struct Array as Input Argument to explain how to handle MATLAB structs in C++ code.

Struct Input

In MATLAB, struct arrays can be created by dynamically assigning field names and values. C++ being a statically typed language requires all fields of a struct and their types to be declared upfront.

When C++ code calls a MATLAB function that requires a MATLAB struct as input, the struct must be explicitly created in C++ using appropriate data types through the MATLAB Data API. For example:

• An ArrayFactory object named factory is created. This object is used to create MATLAB data arrays in C++. It's a factory class that provides methods to create various types of MATLAB data arrays.
• The matlab::data::TypedArray<T> class in the MATLAB Data API is a template class designed to represent arrays of a specific type <T> in C++ code that interact with MATLAB. This class allows C++ programs to create arrays that are compatible with MATLAB data types, facilitating the exchange of data between C++ and MATLAB environments.
  Two TypedArray<double> objects,temperatures and pressures, are created using the factory object. Each call tofactory.createArray<double>() specifies the array dimensions and initializes the array with given values. Here, both arrays are 1-dimensional with 5 elements each, representing temperature and pressure data, respectively.
• A StructArray named inputStruct is created, again using the factory object. ThecreateStructArray method is called with two arguments:
  • The first specifies the dimensions of the struct array. In this case, a 1x1 struct array.
  • The second is an initializer list of strings that define the field names of the struct:"temperatures" and"pressures".
• Finally, the fields of the first (and only) struct ininputStruct are populated with thetemperatures and pressures arrays. The syntax inputStruct[0]["temperatures"] accesses the "temperatures" field of the first struct and assigns the temperatures array to it. Similarly, the"pressures" field is populated with thepressures array.

Struct Output

When a MATLAB function called from C++ returns a MATLAB struct, it is returned as a matlab::data::Array in the C++ code. To retrieve the field names and field values, some processing is necessary. For example:

// Function to print the MATLAB computation results void printMatlabResults(matlab::data::Array outputArray) { matlab::data::StructArray structArray = outputArray; auto topLevelStructFieldNamesIterable = structArray.getFieldNames(); std::vectormatlab::data::MATLABFieldIdentifier topLevelStructFieldNames(topLevelStructFieldNamesIterable.begin(), topLevelStructFieldNamesIterable.end());

for (const matlab::data::MATLABFieldIdentifier& fieldName : topLevelStructFieldNames) {
    std::string outerFieldName(fieldName);
    std::cout << "Field: " << outerFieldName << std::endl;

    matlab::data::TypedArrayRef<matlab::data::Struct> nestedStruct = outputArray[0][fieldName];
    
    auto nestedStructFieldNamesIterable = nestedStruct.getFieldNames();
    std::vector<matlab::data::MATLABFieldIdentifier> 
        nestedStructFieldNames(nestedStructFieldNamesIterable.begin(), nestedStructFieldNamesIterable.end());
    for (const matlab::data::MATLABFieldIdentifier& fieldName : nestedStructFieldNames) {
        std::string innerFieldName(fieldName);
        matlab::data::TypedArrayRef<double> fieldValue = nestedStruct[0][fieldName];

        double value = fieldValue[0];
        std::cout << "  " << innerFieldName << ": " << std::fixed << std::setprecision(4) << value << std::endl;
    }
}

}

• The MATLAB struct returned from the MATLAB function as a matlab::data::Array needs to be converted to a matlab::data::StructArray. Direct access to the struct fields is not available.
  matlab::data::StructArray structArray = outputArray;
• Retrieve the iterable of field names from the struct array.
  auto topLevelStructFieldNamesIterable = structArray.getFieldNames();
  std::vectormatlab::data::MATLABFieldIdentifier
  topLevelStructFieldNames(topLevelStructFieldNamesIterable.begin(), topLevelStructFieldNamesIterable.end());
• Use a loop to process each field in the struct. This involves accessing each field by name and processing its contents.
  for (const matlab::data::MATLABFieldIdentifier& fieldName : topLevelStructFieldNames) {
  std::string outerFieldName(fieldName);
  std::cout << "Field: " << outerFieldName << std::endl;
  ...
  }
• If the fields themselves contain structs, repeat the process of getting field names and iterating over them.
  matlab::data::TypedArrayRefmatlab::data::Struct nestedStruct = outputArray[0][fieldName];
  auto nestedStructFieldNamesIterable = nestedStruct.getFieldNames();
  std::vectormatlab::data::MATLABFieldIdentifier
  nestedStructFieldNames(nestedStructFieldNamesIterable.begin(), nestedStructFieldNamesIterable.end());

See Also

Topics

• Deploy MATLAB Function That Accepts Struct Array as Input Argument