[Rcpp-devel] Setting R seed from Cpp (continued)

[Simon Zehnder]

Hi Matteo,

this is an interesting post. My assumption is, that as in the first approach you call an R function which is implemented as a pointer (or reference to an R function) this function always has access to the RNG scope. Whereas in the second approach you call nested, which is a CPP-function that implicitly calls the R function (via the R namespace in Rcpp I think). In this case the RNG scope is not accessible and must be made accessible via a further 'RNGScope;' in the function 'nested'. What happens if you do this?


Best

Simon


On Jun 10, 2013, at 7:52 PM, Matteo Fasiolo <matteo.fasiolo at gmail.com> wrote:

> Hi all,
> 
>  I continued experimenting with setting the R seed from an 
> Rcpp function. While this code works:
> 
> cppFunction('
>   
> List myFun(int n, Function foo)
> {
>     RNGScope scope;
> 
>     Environment g = Environment::global_env();
>     Environment::Binding RandomSeed = g[".Random.seed"];
>     IntegerVector someVariable = RandomSeed;
> 
>     NumericVector output1(n);
>     NumericVector output2(n);
>     
>     output1 = foo(n);
> 
>     RandomSeed = someVariable;
>     output2 = foo(n);
> 
>     return List::create(output1, output2);
> }
>   
> ')
> 
> myRnorm <- function(n)
> {
>   rnorm(n)
> }
> 
> set.seed(524514)
> myFun(10, myRnorm)
> 
> [[1]]
>  [1] -0.46949481 -0.40449904  1.10377225 -0.94267114  0.71828154  0.44722386 -0.04653023  0.49021681 -0.68894641
> [10]  0.15505136
> 
> [[2]]
>  [1] -0.46949481 -0.40449904  1.10377225 -0.94267114  0.71828154  0.44722386 -0.04653023  0.49021681 -0.68894641
> [10]  0.15505136
> 
> If I put another layer of C++:
> 
> 
> cppFunction('
> NumericVector nested(int n)
> {
>   return rnorm(n);
> }
> ')
> 
> myRnorm <- function(n)
> {
>   nested(n)
> }
> 
> set.seed(524514)
> myFun(10, myRnorm)
> 
> [[1]]
>  [1] -0.46949481 -0.40449904  1.10377225 -0.94267114  0.71828154  0.44722386 -0.04653023  0.49021681 -0.68894641
> [10]  0.15505136
> 
> [[2]]
>  [1] -1.58817318  0.32828207  0.01351185 -0.28672471  0.51732449 -0.10360312  0.97944543 -0.16814832 -0.83571146
> [10] -0.43258866
> 
> This doesn't work any more. Luckily I listened to Dirk's warning about messing with R RNG,
> because this strange "sandwich" (C++ calls R calls C++) is pretty much what I was going to 
> do in my application.
> 
> Matteo
> 
>  
> 
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