The reason you get this error with global (or persistent) variables is that all global/persistent variables are implicitly initialized to []. Since the types need to match during subscripted assignment you get the error. In order to eliminate this error you can:
1. Avoid using the subscript (since the subscripted assignment preserves the original type)
2. Initialize d to be a struct. In this case you could use:
global d;
if isempty(d)
d = struct([]);
end
The basic idea behind suggestion 2 is that you need to create the variable to be a data type in the class of your choice. Then subscripted assignments into it will produce the results you'd like.
