DO NOT USE GLOBALS.
You could avoid the whole problem by not using globals. Just because some beginners like to use global variables does not make them a good way to code: it makes debugging very difficult, and produces bugs that are quite opaque. Avoid globals, and simply pass your data using any of the preferred and reccomended methods given in the MATLAB documentation: passing arguments, nested functions, etc.:
In any case, your bug is easy to figure out, once you actually read the global documentation, which clearly states that "if the global variable does not exist the first time you issue the global statement, it is initialized to an empty 0x0 matrix."
In your first function the global does not exist at the point where you declare it to be global, then MATLAB creates it as an empty matrix (of the default class double). Clearly an empty matrix does not have elements one to six, and is not a structure, so you attempt to force a structure into it is an error. So, you are basically doing this:
>> A = [];
>> A(1:6) = struct('x',1)
??? The following error occurred converting from struct to double:
Error using ==> double
Conversion to double from struct is not possible.
The solution? Avoid using globals. Pass you data properly.
