GNU Octave: Global Variables

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### 7.1 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. Issuing a ‘`clear gvar`’ command does not change the above behavior, but ‘`clear all`’ does.

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;
    endfunction
    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;
    endfunction

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
    endfunction

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 true if name is a globally visible variable.

  For example:

      global x
      isglobal ("x")
         ⇒ 1

  See also: isvarname, exist.

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