structure usage in public function in App-designer

I met a problem while assigning a variable to structure in App-designer:

methods (Access = public)
    function plottest(app,WorkingDir)    
        tmp=3;
        result.a=tmp;
    end
end

but Matlab reported error: The expression to the left of the equals sign is not a valid target for an assignment.

however, when I revise it like this:

methods (Access = public)
    function plottest(app,WorkingDir)    
        tmp=3;
        result=tmp;
    end
end

the problem does not exist anymore.

The reason I want to use structure is that, I have lots of variables that need to save at the end of this function, saving them one by one works but not as well as saving them into one structure.

just want know what is the problem here. is there anyway to resolve it?

Thanks!

Yu