From ripley at stats.ox.ac.uk  Fri Dec  6 16:42:37 2013
From: ripley at stats.ox.ac.uk (Prof Brian Ripley)
Date: Fri, 06 Dec 2013 15:42:37 +0000
Subject: [Rd] Matrix memory layout R vs. C
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References: <52A1CF46.7070408@googlemail.com>	<52A1E17B.8090200@gmail.com>
	<CABtg=KnYfx_VYvGdmximPm7GSSj-eY-vzdcYKpPPtHDcd1ppXg@mail.gmail.com>
Message-ID: <52A1F06D.3080701@stats.ox.ac.uk>

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.

-- 
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)
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[Rd] Matrix memory layout R vs. C
Prof Brian Ripley