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Re: Checking existance of global variable in function?

From: John W. Eaton
Subject: Re: Checking existance of global variable in function?
Date: Wed, 19 Jul 2006 15:05:41 -0400

On 16-Jul-2006, Etienne Grossmann wrote:

|   Btw, this is more a address@hidden mail, iigic.
| On Sun, Jul 16, 2006 at 10:16:24AM -0400, Etienne Grossmann wrote:
| # 
| #   Hi Volker,
| # 
| # what about testing whether graphdir is a string, as in:
| # 
| # function [out] = plotinit (eps, file, gnuplotfile)
| # global graphdir
| # if (exist('graphdir') ~= 0)
| #   if ~ischar(graphdir), graphdir = ''; end
| #   graphdir=[graphdir '']

If a variable is declared global, it will exist, so the 

  if (exist('graphdir') ~= 0)

line is not really needed.

Globals are initialized to [] by default for compatibility with
Matlab.  Octave also allows initialization in the global statement:

  global graphdir = "/some/default/dir";

but that does not work with Matlab.  If you need your code to run in
both systems, then you are stuck with the initial value being [].  If
you need to know whether a global variable has been initialized and
a valid value for the global variable is [] and this is not the
initial value you desire, then you need a auxiliary variable to keep
track of whether the global has been initialized.  For example,
something like

  global foo;
  persistent initialized;

  if (isempty (intialized))
    foo = some_function ();
    initialized = true;

should work.


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