Re: testing global variable from within a function

[Juan Pablo Carbajal]

On Sun, Jan 27, 2013 at 9:08 PM, ernst <address@hidden> wrote:
> Hi Juan,
> Thanks for the quick answer, but my problem remains.
> Let me describe the situation more clearly:
>
> I have a global variable,
>
> global x=1;% defined in octaverc
>
> but is seems as if the function zeros() is invoked (by octave itself) before
> the global settings take effect.
> So I have to distinguish, whether zeros() is invoked by the user after
> initialization
> or before by octave itself.
>
> My naive idea was to check whether x is defined.
> So i tried exist('x','var') from within function zeros().
>
> Then i realized that this returns false unless i declare global x within
> zeros().
> But then it returns true because then, at least it seems to me so,
> octave interprets global x as defining x global.
>
> Ok, so i cannot check whether it is defined or not using exist:
> After declaration global x within zeros(), the variable x exists, before it
> doesn't.
>
> Any idea what do do?
>
> Greetings.
>
> Ernst
>
>
>
>
>
>
>
>
> On Sun, Jan 27, 2013 at 7:51 PM, ernst <address@hidden> wrote:
>
> Hi all,
> I must check from within a function, whether a certain global variable
> 'myvarname' is defined.
>
> What i know is, that from outside the function i just try
> exist('myvarname', 'var').
> From within a function this does not work and i think
> this is because I shall declare the variable global before.
> But i have the impression, that 'global' defines a global variable, if
> it is not yet defined,
> otherwise makes the global variable already defined available.
> So, testing whether it is global, does not make any sense within a
> function,
> because testing after declaring global returns true, testing before
> returns false.
>
> What can i do?
>
> greetings, Ernst
>
> _______________________________________________
> Help-octave mailing list
> address@hidden
> https://mailman.cae.wisc.edu/listinfo/help-octave
>
> "global" is a funny keyword in Matlab and therefore Octave.
>
> For each scope you want a global to be "visible" you have to declare
> it as global.
> The following code would leave g2 untouched, though some people would
> expect it to change
>
> function y = test_global(x)
>  global g1
>  g2= x;
>  y = g1;
> endfunction
>
> global g1 g2
> g2=2;
> g1=-1;
> test_global(-2)
>
> ans = -1
>
> g2
>
> g2 = 2
>
> If you want to access the global g2 you need to tell test_global that
> g2 is a global variable.
>
>
> _______________________________________________
> Help-octave mailing list
> address@hidden
> https://mailman.cae.wisc.edu/listinfo/help-octave
>

What is it that you want to achieve? maybe there is a better solution
than using a global.

In any case, it really doesn't matter when the global variable is
declared/defined. As I interpret it (not sure this is how it is
implemented) when you say "global x" you are "saying use the variable
x from the global scope. If it is not there define it, but empty.". So
if you think that inside your function, x is declared global before it
is assigned a value (as in your .octaverc), then check whether the
variable is empty.

global x;
x = 1;

function myfunc ()
  global x
  if isempty (x)
    printf ("I just declared x as global\n");
  else
    printf ("Global varible X has value: %g\n",x);
  endif
endfunction

myfunc ()

That should print the value of x while the following, (after a "clear
all" or restartgin Octave)
global x;

function myfunc (y)
  global x
  if isempty (x)
    printf ("I just declared x as global\n");
  else
    printf ("Global varible X has value: %g\n",x);
  endif
endfunction

myfunc ()

Should say that ti decleared x as global. However note that this
prints the value of X

global x;

function myfunc (y)
  global x
  if isempty (x)
    printf ("I just declared x as global\n");
  else
    printf ("Global varible X has value: %g\n",x);
  endif
endfunction

x = 1;
myfunc ()

Hope that helps,

JPi