Matrix memory layout R vs. C
On 06/12/2013 15:49, G?bor Cs?rdi wrote:
On Fri, Dec 6, 2013 at 10:42 AM, Prof Brian Ripley <ripley at stats.ox.ac.uk> wrote:
On 06/12/2013 14:42, G?bor Cs?rdi wrote:
On Fri, Dec 6, 2013 at 9:38 AM, Duncan Murdoch <murdoch.duncan at gmail.com> wrote:
On 06/12/2013 8:21 AM, Larissa Hauer wrote:
[...]
I would not assume that a 2D matrix in C doesn't have gaps in it between the rows. Let C treat it as a vector, and write a little macro that does the indexing. For example, #define INDEX(i,j) (i) + rows*(j)
I would make this #define INDEX(i,j) ((i) + rows*(j)) just to be on the safe side.
And to be safer on a 64-bit platform #define INDEX(i,j) ((i) + rows*(R_xlen_t)(j)) since rows*j might overflow there.
Indeed. Of course this still does not save you from indexing out-of-range and integer overflow in the addition.
To a large extent it does. It means the arithmetic is done in R_xlen_t, 64-bit on 64-bit machines, and so any legitimate index gets computed correctly.
Gabor [...]
Brian D. Ripley, ripley at stats.ox.ac.uk Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595