Cant read triangulation from struct with the C API

I want to read in a triangulation from a .mat-file for a tool that is outside of Matlab. When I try to read the following struct :

with the following code which just iterates over all fields in a struct and prints the field names and their class name if the field was not empty

// Print the class of each field in a structure
void analyzeStructArray(const mxArray *struct_ptr) {
        
    mwSize n_fields;         /* number of fields */
    mwSize n_elements;       /* number of elements in array */
    mwIndex fIdx;           /* field index */
    const char *fname;      /* field name */
    const mxArray *fPtr;    /* field pointer */
    mxClassID category;     /* class ID */
    
    n_fields = mxGetNumberOfFields(struct_ptr);
    
    for (mwSize field_id = 0; field_id < n_fields; field_id++) {

        fname = mxGetFieldNameByNumber(struct_ptr, field_id);
        fPtr = mxGetFieldByNumber(struct_ptr, 0, field_id);
    
        if (fPtr == NULL) {
            printf("Empty field: %s\n", fname);
        } else {
            auto class_id = mxGetClassID(fPtr);
            const char * class_name = mxGetClassName(fPtr);
            printf("  %s: of class %s\n", fname, class_name);
        }
    }
}

I get the following output:

  5 fields; 1 elements        
  prob_net: of class double   
  labels_net: of class logical
  labels: of class logical    
Empty field: tri
Empty field: name

I just rechecked the .mat-File in Matlab, but fields are not empty. Could someone explain to me why this happens and how I can access those fields?

[Harimurali]

Hi Pascal,

Could you share the .mat file from which you are trying to read the triangulation?

Answer 1

[Sugandhi]

Hi,

I understand that you are not able to read triangulation from struct with the C API.

mxArray *mxGetFieldByNumber(const mxArray *pm, mwIndex index, int fieldnumber); 

Above code line returns pointer to the ‘mxArray’ in the specified field for the desired element, on success. Returns NULL in C if passed an invalid argument or if there is no value assigned to the specified field. Common causes of failure include:

1. Specifying an array pointer pm that does not point to a structure mxArray. Call ‘mxIsStruct’ to determine whether pm points to a structure ‘mxArray’.
2. Specifying an index to an element outside the bounds of the ‘mxArray’. For example, given a structure ‘mxArray’ that contains 10 elements, you cannot specify an index greater than 9 in C.
3. Specifying a non-existent field number. Call ‘mxGetFieldNumber’ to determine the field number that corresponds to a given field name.

One of the possible workarounds could be checking if there is no value at index 0 assigned to the ‘tri’ field and update it.

For more understanding, kindly go through following link:

• ‘mxGetFieldByNumber’: https://www.mathworks.com/help/matlab/apiref/mxgetfieldbynumber.html?&s_tid=srchtitle_support_results_1_mxGetFieldByNumber