[Rd] Matrix memory layout R vs. C

[Gábor Csárdi]

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, Gábor Csárdi 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.

Gabor

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