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 2018-9-19
回答： Guillaume 2018-9-19
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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Guillaume
Guillaume 2018-9-19
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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