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

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Jesse Hopkins
Jesse Hopkins on 19 Sep 2018
Answered: Guillaume on 19 Sep 2018
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?

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Accepted Answer

Guillaume
Guillaume on 19 Sep 2018
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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