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Re: global variables in .oct-files

From: John W. Eaton
Subject: Re: global variables in .oct-files
Date: Tue, 7 Oct 1997 15:22:34 -0500

On  6-Oct-1997, Stef Pillaert <address@hidden> wrote:

| At 01:02 2/10/97 -0500, you wrote:
| >On 27-Sep-1997, Stef Pillaert BK <address@hidden> wrote:
| >
| >| I'm trying to translate some of my octave-functions (.m-files) into C++
| >| (.oct-files) to speed up things. I use a lot of global variables though,
| >| otherwise I have to pass a lot of parameters to each function. Is there a
| >| way to "see" those global variables in my C++-code?
| >
| >Yes, it's possible.  Here is an example of how you can get and set
| >global values in .oct files:
| >
| > [...]

| This works great, but what I want to do now is: just changing a few
| elements in a large (global) variable. So I'm really looking for a
| translation of what in a .m-file would be:
| function f1()
| global matrixa
| matrixa(1,1)=10000;
| end
| (yes, I actually use a function as if it was a "procedure").
| I tried to translate this into C++ with something like:
|    octave_value val=get_global_value("matrixa");
|    Matrix matrixb = val.matrix_value();
|    matrixb(1,1) = 10000;      
|    octave_value val2=octave_value(matrixb);
|    set_global_value("matrixa",val2);
| This doesn't work (octave crashes on it),

It should work, but you have to remember that the C++ Matrix type uses
zero-based indexing.  So an equivalent function would be something
like this (using the definitions of get_global_value and
set_global_value as before):

  DEFUN_DLD (foo, , ,
    "foo ()")
    octave_value_list retval;

    octave_value val = get_global_value ("a");

    if (val.is_defined ())
        Matrix a = val.matrix_value ();

        // Might also want to include checks on the dimensions here...

        a(0,0) = 10000;

        set_global_value ("a", a);
      error ("unable to find symbol `a' in global scope!");

    return retval;

  Octave, version 2.0.9 (i686-pc-linux-gnu).
  Copyright (C) 1996, 1997 John W. Eaton.
  This is free software with ABSOLUTELY NO WARRANTY.
  For details, type `warranty'.

  octave:1> a = zeros (3);
  octave:2> foo
  error: get_global_value: unknown symbol `a'
  error: unable to find symbol `a' in global scope!
  error: evaluating expression near line 2, column 1
  octave:2> global a
  octave:3> foo
  octave:4> a
  a =

    10000      0      0
        0      0      0
        0      0      0

| and even if it would work, I presume there is an unnecessary copy of
| the  global variable. I tried to write a C++ function
| "change_global_value" (or should it be
| "change_some_elements_in_global_value" ?), but I'm affraid my
| knowledge of C++ is to limited and I get into trouble with pointers
| and stuff... Anyone with a few minutes of time to help me out?

Yes, the above function creates a second copy of the matrix when the
indexed assignment operation is executed.

Unfortunately, there's currently no good way to avoid this problem.
I've added a note about it to the PROJECTS file.



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