[Rcpp-devel] Persistence of C++ changes in R.

[Cedric Ginestet]

Thanks Douglas and Dirk,

Just to answer Douglas: No, I didn't get an error message when compiling 
the cxx.function that I posted yesterday. Regarding the use of clone, is 
it more time consuming to use this particular command?
Cheers,

Here are the corrected codes. It now works fine, or rather as I expected 
from my experience with R.
###############################################
D <- matrix(rnorm(4),2,2)
src3 <- '
   NumericMatrix xD(clone(D));
   xD(0,0) = 100.0;
return xD;
'
passRef <- cxxfunction(signature(D="matrix"),body=src3,plugin="Rcpp")
passRef(D); D
##############################################



On 22/12/10 18:32, Douglas Bates wrote:
> On Wed, Dec 22, 2010 at 12:08 PM, Cedric Ginestet
> <c.ginestet05 at googlemail.com>  wrote:
>> Dear Rcpp Experts,
>>
>> I have encountered two issues when modifying an object within C++ function.
>> See the code below:
>>
>> ##########################################
>> library(inline); library(Rcpp)
>> D<- matrix(rnorm(4),2,2)
>> src3<- '
>>    NumericMatrix xD(D);
>>    xD[1,1] = 100.0;
>> return xD;
>> '
>> passRef<- cxxfunction(signature(D="matrix"),body=src3,plugin="Rcpp")
>> passRef(D); D
>> ###########################################
>>
>>  From which I get the following:
>>             [,1]      [,2]
>> [1,]  -0.587375 0.3173949
>> [2,] 100.000000 0.1801181
>>             [,1]      [,2]
>> [1,]  -0.587375 0.3173949
>> [2,] 100.000000 0.1801181
>>
>> I have two questions here:
>> 1. Why is the [2,1] modified, when I would have expected the [2,2] entry to
>> be modified, given that I had assign 100.0 to xD[1,1] in C++?
> Well when I call cxxfunction on that code I get a warning.  Did you?
>
> The warning tells me that I am indexing xD as a one-dimensional array
> and that leads me to believe that I probably want xD(1,1), not xD[1,1]
>
>> 2. Why does the change to the object D persist when we call the object
>> again? This looks like D has been passed by reference and is therefore
>> affected by what happened within the function. However, this is not the
>> expected behaviour for an R function. What's going on here?
> It is the expected behavior when you pass an R object through the
> .Call interface.  If you want to change a copy then define
>
> NumericMatrix xD(clone(D));
>
> which does the deep copy operation.
>> Thank you very much for your help,
>> Cedric
>>
>>
>> --
>> Cedric Ginestet
>> Centre for Neuroimaging Sciences (L3.04)
>> NIHR Biomedical Research Centre
>> Department of Neuroimaging
>> Institute of Psychiatry, Box P089
>> King's College London
>> De Crespigny Park
>> London
>> SE5 8AF
>> Tel: (+44) 20-3228-3052
>> Fax: (+44) 20-3228-2116
>> Email: cedric.ginestet at kcl.ac.uk
>>
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>>