Hi,
I'm learning C and I saw in a book that a function prototype has the form void f() and in the function declaration or in the calling function, the f function takes arguments.
Thus In the function declaration we have something like void f(long double y[], long double A) and in the calling function is f(y, A).
The function is doing operations on the array y i.e. when the function is called, some elements in the array y are changing. A is just a constant numerical value that doesn't change. I have two questions:

1. If defining the function prototype at the top in the program as void f() a good practice? Or is it better to put it as void f(long double y[], long double A) as in the function declaration?
   

2. The called function f is changing elements in the array y. Is void the right return type? The program is working fine as such with the void as described.
   Or should I change all my "voids" to "long double".
   I'm working with long double as I need as much precision as possible though on my machine both double and long double gives me 15 precision digits.
   

Thanks a lot