Rik and John,

I just found that there has been a change in the handling of global since 16 Jun. The change in this report could be the cause.

With a system build on 16 June I could reference a global
in a script which did not itself contain a global declaration.
Now that does not work. It is necessary to declare the global
in the script even though the variable is shown as existing
(as global) in a whos before invoking the script.

This change may be intentional and I would agree if it was
intended. Just thought it would be good to point out the
change.

I pushed the following changeset in an attempt to make the warnings clearer about what is happening:

http://hg.savannah.gnu.org/hgweb/octave/rev/0d7a89bec20e

Pushed on stable and merged to default.

Yes, if there is no global value for x defined, then

x = 12
global x

will make x global and preserve the local value.  But if there is already a global x, then the same statements will make x global but not change the global value to match the local value.  I see now that I got the warning message wrong.  I think we can make this behavior somewhat clearer by issuing two different warnings:

warning: global value of 'x' initialized to existing local value

for the first case, and

warning: local value of 'x' changed to match existing global value

for the second.

I believe this behavior is compatible with Matlab's current behavior, but it only issues the one "local value may have changed" warning, which is kind of weird since it should be possible to warn when that actually does occur instead of just that it could.

This introduced a strange behaviour

>> x=12
x =  12
>> global x
warning: global: 'x' is defined in the current scope
warning: global: in a future version, global variables must be declared before use
warning: global: global value overrides local value
>> x
x =  12
>> clear x
>> x=13
x =  13
>> global x
warning: global: 'x' is defined in the current scope
warning: global: in a future version, global variables must be declared before use
>> x
x =  12
>>

So despite of "clear x" the global value of x still persists somewhere...

Matlab R2017b (9.3.0.713579) 64-bit (glnxa64)

does the same as Markus showed for R21016a

Thanks, marking as confirmed, declaring a global variable should inherit its previous value if it was defined already as a non-global.

Michael - thanks, but the test that we need in Matlab is without the declarit function, what is the value of x after it is declared global, does it retain the value of the previous normal variable named x or is it reset to empty?

clear all
x = 12
global x
x

This has been stalled for a week.  Maybe the test code can be posted to the Octave-Maintainers list for someone with Matlab access to test.

Maybe I misunderstood the matlab doc. I don't have matlab to test it. 

But a warning could be useful, indeed. Yesterday I spent at least an hour to locate a problem resulting from a late global in my code...

>> a=1
a =  1
>> global a
>> a
a = [](0x0)
>>

Matlab will retain the previous value instead but issue a warning, according to its documentation.

The Octave documentation does not specify what behaviour to expect.

In addition, I don't understand what the help

>> help global

 -- global VAR
     Declare variables to have global scope.

          global X;
          if (isempty (X))
            x = 1;
          endif

     See also: persistent.

 
intents to explain. It would make some sense to me if the last x was uppercase, too?