Matlab Variable Scope

Variable Scope

In MATLAB the function workspace is independent of the base workspace; the two cannot share variables directly. The best way to communicate between them is through argument passing.

The methods below offer additional ways to bridge workspaces.

Global Variables

Variables in a function workspace are local variables; global variables are their counterpart. To read or modify the same variable from inside and outside a function, declare it global with the global keyword before each access. Without global, two same-named variables remain isolated and cannot be reached from another workspace.

• global syntax: global x1 ... xn
  • Names are separated by whitespace, with no parentheses or commas

Example:

global x
x = 'gloabal';
y = 'base';
fun();
 
function fun()
global x y
disp(x)
disp(y)
end

This prints global but not base, because the function fun has not yet created the global y.

Persistent Variables

A persistent variable is a special local variable. Ordinary local variables are cleared when the function returns; a persistent variable is preserved in the function workspace, so the next call sees the same value the previous call left behind.

• Declare a persistent variable inside a function definition with the persistent keyword; the syntax mirrors global: persistent x1 ... xn
• A persistent variable is initialized to an empty array on its first declaration
• Clearing the function (e.g. clear fun) also clears its persistent variables, since the entire function workspace is cleared

Example:

function findSum(inputvalue)
persistent SUM_X
 
if isempty(SUM_X)
   SUM_X = 0;
end
SUM_X = SUM_X + inputvalue;

Variables in Nested Functions

Variables in nested functions are also a special kind of local variable: they cannot be referenced from the parent function or from sibling nested functions.

A nested function can, however, read variables from the workspace of any of its ancestor functions.