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Subject: Re: Slice from a matrix?
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P S Norby wrote:

> The trouble then is that you can either have rows of columns, in which case
> you can take a slice of a column, but not a slice of a row... or you can
> have columns of rows, in which case you can take a slice of a row, but not a
> slice of a  column.  The col_vector and row_vector are not subtypes of
> matrix_type.

Not that I would ever throw this at a compiler  but this is not true.

Let's define three types, a "conventional" Ada two-dimensional array, a 
derived type from that type which uses Fortran (row major) layout. 
Finally a vector of vectors type that can be sliced, and which can be 
mapped to the other two types using Unchecked_Conversion.

Will it work in GNAT?  I think so.

Do I think it is a good idea?  Hell no.

I'd much rather define Row and Column functions for the two dimensional 
array type.  Three or four lines of code each, and a good optimizer 
might figure out to actually return a fat pointer to the slice for the 
Row operation.