Mex Programming: How can I know the length of a struct array that is a field of a parent struct?

Hello, consider the following matlab structure, with a field that is a structure array of some length:

substructlength = randi(100);

for i = 1:substructlength
    root.foobar(i).a = rand;
end

I want to process 'root' from a mex file. It doesn't seem possible to know the length of the substructure 'foobar' using just the root mxArray input. I see the mxGetField documentation requires an index input, so one must know the maximum index a-priori:

    mxArray *mxGetField(const mxArray *pm, mwIndex index, const char *fieldname);

How can I know the length of the 'foobar' sub-structure array using only the root mxArray?

Answer 1

[Guillaume]

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.