[Rcpp-devel] Function changing the value of objects in global environment

[Dirk Eddelbuettel]

Nick,

On 1 February 2014 at 20:14, Nick Menzies wrote:
| Hi all, 
| 
| I am relatively new to Rcpp and a bit confused about the behavior I am seeing
| from an Rcpp function. It seems that a function created with Rcpp can modify
| the value of objects in the global environment, yet this behavior depended on
| the nature of the object. I realize I could code things differently so that the
| object in the global environment is not affected, but I wanted to make sure
| there is not something more general I am doing wrong (and which will continue
| to be wrong without me knowing it!)

Yes.   It is known and documented as has been discussed several times here.  

In short in ...
 
| Below is a small example where the global value gets altered, followed by an
| example where it does not. The only difference between them is that in the
| first (cTest1) the input is NumericVector, while in the second (cTest2) the
| input is double. 
| 
| ### Rcpp function alters values in global environment:
| cppFunction('  
|   double cTest1( NumericVector zzz) {

... this case zzz is really the same as the R object, so the change has side
effects.  We are after all passing a _pointer_ -- the P in SEXP object.  If
you want to prevent this, use Rcpp::clone(...) on some object which creates a
distinct copy.

|     zzz = zzz/10; 
|     return 1;
|    }')
| 
|   zzz=rep(10,3)
|   zzz
|   cTest1(zzz)
|   zzz 
| 
| ### Rcpp function does not alter values in global environment:
| cppFunction('  
|   double cTest2( double zzz) {

This is different as a double does not exists in R (it would be a vector of
length one, and correspond to a NumericVector as above -- a double really is
different).   So here you have effect of Rcpp::clone() by an implicit copy.

Idem when a cast is involved.

Hope this helps,  Dirk

|      zzz = zzz/10; 
|      return 1;
|    }')
| 
| zzz=10
| cTest2(zzz)
| zzz 
| ####################
| 
| 
| Thanks for your help, and sorry for the naive question. 
| 
| Nick
| 
| PS - I am running Rcpp 0.10.6, R version 3.0.1
| 
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-- 
Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com