From bug-octave-request at bevo dot che dot wisc dot edu Fri Apr  2 14:07:33 2004
Subject: Re: Inconsistency between global and persistent
From: Quentin Spencer <qspencer at ieee dot org>
To: "John W. Eaton" <jwe at bevo dot che dot wisc dot edu>
CC: bug-octave at bevo dot che dot wisc dot edu
Date: Fri, 02 Apr 2004 14:08:18 -0600

I rebuilt octave with these changes, and ran a function with code like this:

persistent x
if(isempty(x))
...
end

and it still complains that 'x' is undefined when it gets to the isempty 
statement.



John W. Eaton wrote:

>On  2-Apr-2004, Quentin Spencer <qspencer at ieee dot org> wrote:
>
>| If I recall correctly, the behavior of global was modified a while ago 
>| to be more MATLAB compatible. For example, declaring a variable to be 
>| global automatically initializes it to an empty matrix. Declaring a 
>| variable to be persistent does not do this. I am trying to write a 
>| function that saves time by only going through the initialization code 
>| the first time the function is called. Here's how I would do this in MATLAB:
>| 
>| persistent x
>| if(isempty(x))
>|     {initialization code here}
>| end
>| 
>| With the current behavior (version 2.1.57) of persistent in octave, 
>| here's how I would do the same thing:
>| 
>| persistent x = {initialization code here}
>| 
>| The problem is that these two cases really can't be reconciled without 
>| having interpreter-specific code for each case. Initially I liked the 
>| octave implementation, but I can see benefits of the MATLAB 
>| implementation if the initialization code is long and involves multiple 
>| steps. Since we already have the MATLAB-like behavior for globals, would 
>| it be reasonable to modify persistent to behave the same way?
>
>Please try the following patch.
>
>Thanks,
>
>jwe
>
>
>2004-04-02  John W. Eaton  <jwe at bevo dot che dot wisc dot edu>
>
>	* pt-decl.cc (tree_static_command::do_init): Initialize to empty
>	matrix by default.
>
>
>Index: pt-decl.cc
>===================================================================
>RCS file: /usr/local/cvsroot/octave/src/pt-decl.cc,v
>retrieving revision 1.18
>diff -u -r1.18 pt-decl.cc
>--- a/pt-decl.cc	13 Jul 2003 23:15:29 -0000	1.18
>+++ b/pt-decl.cc	2 Apr 2004 16:56:43 -0000
> at @ -147,13 +147,18 @@
>     {
>       id->mark_as_static ();
> 
>-      tree_expression *expr = elt.expression ();
>+      octave_lvalue ult = id->lvalue ();
> 
>-      if (expr)
>+      if (ult.is_defined ())
> 	{
>-	  octave_value init_val = expr->rvalue ();
>+	  tree_expression *expr = elt.expression ();
> 
>-	  octave_lvalue ult = id->lvalue ();
>+	  octave_value init_val;
>+
>+	  if (expr)
>+	    init_val = expr->rvalue ();
>+	  else
>+	    init_val = Matrix ();
> 
> 	  ult.assign (octave_value::op_asn_eq, init_val);
> 	}
>
>  
>



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