MATLAB: How to use attribute GLOBAL efficiently

Question

MATLAB Help describes this topic (word "global") very shortly.

I ask it from such question:

M-file as function:

function [y]=y(x) a=2; y=a*x.^2 end

how to force the procedure to modify global variables? that is, after proc. work, some results will be saved in globals and be available for other procedures.

For example, assigning a=2 I wish to keep. But I don't want use simple ways: 1) text file 2) write [y,a] in header, or y=[y; a] in body

If I write global a; a=2

this haven't effect…

and may anybody give good practical code example with and without GLOBAL?

Answer 1

Working with GLOBALs is fragile in all programming languages: It is very hard to find out, where and why the value was set the last time.

Therefore I'd use Paulo's approach ever: "function [y,a]=y(x)" and forward of [a] to all functions, which need it.

Another approach is using a dedicated function to store the value persistently:

function a = Store(Command, a)
persistent a_
switch Command
  case 'get', a = a_;
  case 'set', a_ = a;
  otherwise error('unknown command');
end

Although this has the same drawback as GLOBAL ([a] can be influenced from anywhere), you can at least use the debugger to track changes and usage of [a].

But it is at least possible to use GLOBALs (I avoid using "y" as variable and function name at the same time):

function yValue = y(x)
global a
a = 2;
yValue = a * x .^ 2;
end

Then from the command line or inside another function:

global a
disp(a)   % >> nothing
k = y(1:10);
disp(a)   % >> 2