From octave-maintainers-request at bevo dot che dot wisc dot edu Tue May 14 22:29:23 2002
Subject: cell arrays and structure arrays
From: "John W. Eaton" <jwe at bevo dot che dot wisc dot edu>
To: octave-maintainers mailing list <octave-maintainers at bevo dot che dot wisc dot edu>
Date: Tue, 14 May 2002 22:28:47 -0500

I've just checked in some changes to the Octave CVS archive that make
cell arrays and structure arrays work.  This includes a significant
set of changes to the code that handles indexing and indexed
assignment of values.  There will be some incompatibilities for
existing user-defined data types.  I realize that this may cause some
trouble, but I think the new way is much better than the old and
justifies the backwardly incompatible change.  I also don't think
there are very many user-defined data types out there, and I'm willing
to provide some help with conversions.

Here is a sample of what is now possible (x previously undefined):

  octave:1> X{2,2}.foo(2).bar(2:3,2:3) = rand (2)
  x =
  {
    [1,1] = [](0x0)
    [2,1] = [](0x0)
    [1,2] = [](0x0)
    [2,2] =
    {
      foo =
      {
	bar =
	(
	  [1] = [](0x0)
	  [2] =

	    0.00000  0.00000  0.00000
	    0.00000  0.86394  0.22945
	    0.00000  0.51638  0.37879

	)
      }
    }
  }

In the above, X is a 2x2 cell array, FOO is a 2-element structure
array with one key, BAR, a 3x3 matrix.  Here is what happens after you
add another element (BAZ) to the structure array:

  octave:3> x{2,2}.foo(1).baz = "I am one BAZ";
  octave:4> x{2,2}.foo(2).baz = "I am too BAZ";
  octave:5> x
  x =
  {
    [1,1] = [](0x0)
    [2,1] = [](0x0)
    [1,2] = [](0x0)
    [2,2] =
    {
      foo =
      {
	baz =
	(
	  [1] = I am one BAZ
	  [2] = I am too BAZ
	)
	bar =
	(
	  [1] = [](0x0)
	  [2] =

	    0.00000  0.00000  0.00000
	    0.00000  0.86394  0.22945
	    0.00000  0.51638  0.37879

	)
      }
    }
  }

With these new types, you can create some really complex data
structures.  But more importantly, both are supposed to be Matlab
compatible, so people who want it can provide much more Matlab
functionality than in the past.

I think simple structures are still backwardly compatible with
previous versions of Octave.

Some things that still need to be done:

  * Cell arrays should really be N-dimensional.

  * Given a cell, X, Expressions like X{:} should return a "comma
    separated list" object that can be passed in to a function as a
    series of disticnt arguments, one per list item.  Currently Octave
    produces an Octave list value, which does not have this special
    property (if you pass it to a function, the function sees a single
    list paramter).

  * Save and load need to be fixed to handle structure arrays and cells.

  * The [ ] operator needs to be fixed so that it can be used to
    concatentate cell arrays as well as numeric and character arrays.

  * ?

Comments?

Thanks,

jwe