Re: How to pass two dimensional arrays to C

["Jeffrey R. Carter" <spam.not.jrcarter@acm.not.spam.org>]

Simon Wright wrote:
> 
> I would have thought that, provided you choose the right orientation,
> the rows would be as expected by C (ie contiguous values).
> 
> In
> 
>    type M is array (Integer range <>, Integer range <>) of Float;
>    --               Column            Row
> 
> the Row elements would be contiguous? (I can see there _might_ be
> compiler differences, and ISTR that GNAT has a way of specifying
> Fortran conventions).

The 1st dimension is considered to index the rows and the 2nd, the 
columns (from the mathematical notation for 2D matrices). In Ada, all 
the components of the 1st row are stored in sequence, followed by all 
the components of the next row, and so on. "An implementation should 
normally represent multidimensional arrays in row-major order, 
consistent with the notation used for multidimensional array aggregates 
(see 4.3.3). However, if a pragma Convention(Fortran, ...) applies to a 
multidimensional array type, then column-major order should be used 
instead (see B.5, ``Interfacing with Fortran'')." (ARM 3.6.2). (Note 
that convention FORTRAN is defined by the language, not GNAT specific.)

In other words, the 2nd, or column, index varies most quickly with 
increasing address.

So to my mind, and using the notation I've described here, the row 
elements are contiguous. So I don't know whether we agree or disagree.

-- 
Jeff Carter
"Whatever it is, I'm against it."
Horse Feathers
46