From bug-octave-request at bevo dot che dot wisc dot edu  Tue Nov 18 03:01:57 1997
Subject: Initial value of global variable
From: "John W. Eaton" <jwe at bevo dot che dot wisc dot edu>
To: Maximilian Pitschi <Maximilian dot Pitschi at ofw dot pr dot siemens dot de>
cc: bug-octave at bevo dot che dot wisc dot edu
Date: Tue, 18 Nov 1997 02:59:28 -0600

On 18-Nov-1997, Maximilian Pitschi <Maximilian dot Pitschi at ofw dot pr dot siemens dot de> wrote:

| In matlab, the initial value of a global variable
| is the empty matrix, [].  In octave (with --traditional), such a
| variable is not defined.

If you are using Matlab, what is the proper way to initialize a global
variable only if it doesn't already have a value?  It seems that

  global foo
  if (isempty (foo))
    foo = initial_foo_value;
  end

will not work if `[]' is among the valid values for foo.  For example,

  function f ()
    global foo
    if (isempty (foo))
      foo = 1;
    end
    if (foo == [])
      do_empty_foo_thing ();
    else
      do_nonempty_foo_thing ();
    end

  global foo
  f ()       ## do nonempty foo thing by default.
  foo = []
  f ()       ## I really wanted to do the empty foo thing!

Would I really have to use another variable to keep track of whether
`foo' should be initialized, or am I missing something here?

In any case, I'm considering making this change, along with making
statements like

  global foo = initial_foo_value;

only perform the assignment if the global foo does not already have a
value.  Then I could write the above code like this:

  function f ()
    global foo = 1
    if (foo == [])
      do_empty_foo_thing ();
    else
      do_nonempty_foo_thing ();
    endif
  endfunction

  global foo
  f ()       ## do nonempty foo thing by default.
  foo = []
  f ()       ## do the empty foo thing now.

and get the results I expect.

The unfortunate side-effect is that if you are using global
variables to keep track of some state information that is common to
several subroutines (see Octave's multiplot M-files, for example) and
you write something like

  function f ()
    global foo = 1;
    ...
  end

  function g ()
    global foo = 2;
    ...
  end

you might get unpredictible results depending on the order the
functions are called.

Comments?

Thanks,

jwe