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

[Cedric Ginestet]

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++?
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?

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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