global in Matlab

Question

hi guys im noob at Matlab and i cannot understand global variables. like=> global x y z what does global means? could you give me example? any help will be appreciated.

Answer 1

The keyword GLOBAL in MATLAB lets you define variables in one place and have them visible in another. The statement global x y z declares x y and z to be global. If that definition were placed inside a function scope, and a corresponding global definition in a separate function, then the two functions can share data without the need to pass parameters. Like so:

%% fcn1.m
function fcn1( val )
global store
store = val

%% fcn2.m
function y = fcn2()
global store
y = store;

Thus you can call fcn1 to store a value, and use fcn2 to retrieve the value later.

Using GLOBAL data in this way hides the dependencies between your functions, and is considered pretty bad practice.

Answer 2

Without global, a declared variable has a limited scope, that is, it cannot be used outside the "part" of matlab where it is declared. This wiki article has a decent discussion of what scope is in a generic sense.

For Example, if you declare a variable inside a function definition, then that variable can only be used inside that function - not in another function. If you define a function at the command line (the so-called "base workspace") it cannot be used in functions.

global defines a variable in the "global" scope - it can be used anywhere, in any function, etc. Except in limited cases, it is generally a bad idea, since it makes controlling how and when a variable has changed very difficult. You are usually better off returning something from a function, then passing it to another.