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9.1 Global Variables

A variable that has been declared global may be accessed from within a function body without having to pass it as a formal parameter.

A variable may be declared global using a global declaration statement. The following statements are all global declarations.

     global a
     global a b
     global c = 2
     global d = 3 e f = 5

A global variable may only be initialized once in a global statement. For example, after executing the following code

     global gvar = 1
     global gvar = 2

the value of the global variable gvar is 1, not 2.

It is necessary declare a variable as global within a function body in order to access it. For example,

     global x
     function f ()
       x = 1;
     f ()

does not set the value of the global variable x to 1. In order to change the value of the global variable x, you must also declare it to be global within the function body, like this

     function f ()
       global x;
       x = 1;

Passing a global variable in a function parameter list will make a local copy and not modify the global value. For example, given the function

     function f (x)
       x = 0

and the definition of x as a global variable at the top level,

     global x = 13

the expression

     f (x)

will display the value of x from inside the function as 0, but the value of x at the top level remains unchanged, because the function works with a copy of its argument.

— Built-in Function: isglobal (name)

Return 1 if name is globally visible. Otherwise, return 0. For example,

          global x
          isglobal ("x")
               => 1