Re: cell arrays and structure arrays

[Paul Kienzle]

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

There is something to be said for moving concatenation into the operator
table rather than hard coding it into tree_matrix.  Then it would be
natural to construct block sparse matrices and symbolic matrices.  The
following comment comes from symbolic.html:

   Limitations/Features
   Currently  there is no support for symbolic matrices. I think it would
   require  a  few  changes  to  the parser to do it nicely: for example:
   sym_matrix  =  [x+1,  x+5;  x^2+4,x^2+2*x+1];  I could make a function
   like  sym_matrix(the_rows,the_columns,x+1,  ...  )   that  returned  a
   symbolic matrix but this would be a bit of a kludge.

Indeed, matlab provides horzcat and vertcat methods for objects.  

It would be nice to be able to precalculate the size and type of the
resulting matrix before starting any concatenation so that you are not
resizing the matrix every step of the way.  You won't be able to do
that with just horzcat and vertcat.  Instead you need some way to
precalculate the resulting type and dimension, build a new matrix
of that type, and insert appropriate octave values into the indices
of that matrix.  Then you could write tree_matrix as follows:

precalculate
        r = 0
        for each horizontal cat in vertical cat
            c = 0
            rows = firstvalue->rows()
            type = firstvalue->type()
            for each octave value in horizontal cat
                horztype = cancat(horztype, value->type())
                if rows != value->rows() error
                c += value->columns();
            end
            if first
                verttype = horztype
                cols = c
            else
                verttype = cancat(verttype,horztype)
                if cols != c error
            end
            r += rows;
        end

construct
        octave_value ret(type,r,c);
        r = 0
        for each horizontal cat in vertical cat
            c = 0
            rows = firstvalue->rows();
            for each octave value in horizontal cat
                ret->insert(r,c,value);
                c += value->columns();
            end
            r += rows;
        end

With automatic type conversion, my code will do something different than a
series of horzcat/vertcat calls.  Consider the following:


        [ 1, 2, { 3, 4} ]

With horzcat, this is

        horzcat(horzcat(1,2), { 3, 4 } )

which would be

        { [1, 2], 3, 4 }

But as I've written it, this will be

        { 1, 2, 3, 4 }

Maybe that's why matlab says:

        >> [ 1, 2, { 3, 4} ]
        ??? Error using ==> horzcat
        Conversion to cell from double is not possible.


- Paul