How can I pass a struct that contains different numeric arrays into a MEX function and have its fields accessed?

It is sought to use a struct in MATLAB that can be filled with an arbitrary number of numeric arrays as elements, while subsequently pass this array to a MEX-function and have all its fields accessed.

How can I pass a struct that contains different numeric arrays into a MEX function and have its fields accessed?

Answer 1

[MathWorks Support Team]

This workflow can be achieved by using such MEX-API functions as, 'mxIsStruct' to check if a mxArray is a struct, 'mxGetNumberOfFields' to get the number of fields in case the mxArray represents a struct, 'mxGetFieldNameByNumber' to get the name of the field of a mxArray that represents a struct, 'mxGetFieldByNumber' to get the content in a particular element of a struct array of a particular field of a mxArray that represents a struct, and 'mxGetNumberOfElements' to get the number of elements that a mxArray has. As an example the following implementation of such a MEX-function is herein provided,

/*
 * structArrays.c
 *
 * This is a MEX file for MATLAB.
*/

#include "mex.h"
#include "matrix.h"

/* The gateway function */
void mexFunction(int nlhs, mxArray *plhs[],
                 int nrhs, const mxArray *prhs[])
{
    /* Print out the number of input arguments to the MEX-function */
    mexPrintf("Number of input arguments %d\n", nrhs);

    /* loop over all the input arguments to the MEX-function */
    for (mwSize iEl = 0; iEl < nrhs; ++iEl) {
        /* Get the mxArray corresponding to the current input */
        const mxArray* my_el = prhs[iEl];
    
        /* Check if the mxArray is a struct */
        bool isStruct = mxIsStruct(my_el);
        if (isStruct) {
            /* Get the number of fields */
            int numFields = mxGetNumberOfFields(my_el);
            mexPrintf("Number of fields %d\n", numFields);

            /* loop over all number of fields*/
            for (mwSize iFields = 0; iFields < numFields; ++iFields) {
                const char* namefield = mxGetFieldNameByNumber(my_el, iFields);
                mexPrintf("Field name: %s\n", namefield);

                /* Get mex-array */
                const mxArray *my_arr = mxGetFieldByNumber(my_el, 0, iFields);

                /* Check if mex-array represents a double field */
                bool isDouble = mxIsDouble(my_arr);
                if (isDouble) {
                    mwSize numEl = mxGetNumberOfElements(my_arr);
                    mxDouble *Value = mxGetDoubles(my_arr);
                    for (mwSize i = 0; i < numEl; ++i)
                        mexPrintf("%f ", Value[i]);
                    mexPrintf("\n\n");
                }
            }

        }
        
    }
}

Compile the latter mex-file using the following command,

 

mex structArrays.c -R2018a

where flag '-R2018a' is needed since API function 'mxGetDoubles' is used, see also the following documentation page for more information,

https://de.mathworks.com/help/matlab/apiref/mxgetdoubles.html

Then, please use the following MATLAB code for the proof of concept,

 

my_struct.arr1 = [1 2; 3 4];
my_struct.arr2 = [1 2 3; 4 5 6];
my_struct.arr3 = [1 2 3; 4 5 6; 7 8 9];

structArrays(my_struct)

where 'structArrays' stands for the name of the MEX-file hosting the latter implementation, to receive the following output,

 

>> my_struct.arr1 = [1 2; 3 4];
my_struct.arr2 = [1 2 3; 4 5 6];
my_struct.arr3 = [1 2 3; 4 5 6; 7 8 9];
>> structArrays(my_struct)
Number of input arguments 1
Number of fields 3
Field name: arr1
1.000000 3.000000 2.000000 4.000000 

Field name: arr2
1.000000 4.000000 2.000000 5.000000 3.000000 6.000000 

Field name: arr3
1.000000 4.000000 7.000000 2.000000 5.000000 8.000000 3.000000 6.000000 9.000000