MATLAB: How to access a single element of a structure array using the MATLAB mxArray interface in C

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I would like to extract a single element from a structure array using the MATLAB mxArray interface provided in C.

Best Answer

The ability to extract a single element from a structure array is not present using C MX-functions. To work around this issue, you can access sections of the structure array using the "mxGetData" command. An example of this follows:

mxArray **element_search
mxArray *mystruct;
mystruct = createarray();
element_search  = (mxArray **)mxGetData(mystruct);

In the above code "mystruct" contains the contents of a MATLAB structure array. The function "createarray" populates the structure with data. The data found in the structure is an array containing variables of type "mxArray *". To access the fields of a specific element, you would access the variable using the syntax

element_search[field + element*numFields];

where "field" is the field number, "element" is the element number you wish to locate and "numFields" is the number of fields in each element.

Related Solutions

MATLAB: How to access nested MATLAB structure fields in the MEX program

The following code fragment demonstrates how to extract a substructure and then extract a field from that substructure:

substructure_field_num = mxGetFieldNumber(pa, "substructure");
mxArray *sub = mxGetFieldByNumber(pa, index, substructure_field_num);
field_num = mxGetFieldNumber(sub, "my_field");
mxArray *myField = mxGetFieldByNumber(sub, index, field_num);

For more information on using structures in general, see the documentation for mxGetFieldByNumber and the related PHONEBOOK.C example. You can use the following command in MATLAB to access the documentation:

 doc mxGetFieldByNumber

MATLAB: Mex Programming: How to know the length of a struct array that is a field of a parent struct

You misunderstood what the index refers to. It's the index of the parent structure, not of the field. So for structure

root(i1).foobar(i2).a

You'd call

mxArray* fn = mxGetField(root, i1, 'foobar');  %access field of element i1 of root

which gives you a pointer to another structure array for which you can query the size with mxgetNumberOfElements and co.

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Related Solutions

MATLAB: How to access nested MATLAB structure fields in the MEX program

MATLAB: Mex Programming: How to know the length of a struct array that is a field of a parent struct

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