[Rcpp-devel] Problem passing Armadillo objects back to R

[Dirk Eddelbuettel]

Hi Guenter,

On 7 March 2011 at 16:03, "Günter J. Hitsch" wrote:
| 
| I'm new to Rcpp and RcppArmadillo---so far I like it a lot!  Thanks to the
| developers for their good work.
| 
| I run into a peculiar kind of problem when I try to pass a "large" Armadillo
| object back to R.  Here's some code to replicate the problem in stylized form:
| 
| 
| extern "C" SEXP testFun(SEXP L_)
| {
| const long L  = Rcpp::as<long>(L_);
| arma::mat X(L,1);
| arma::mat Y(L,1);
| X.fill(1);
| Y.fill(2);
| return Rcpp::List::create(
| Rcpp::Named("X") = Rcpp::wrap(X),
| Rcpp::Named("Y") = Rcpp::wrap(Y)
| );
| }
| 
| 
| I compile (both using g++ and the Intel Compiler --- choice of compiler makes
| no difference) and then call from R:
| 
| 
| L = 1000000
| ret  = .Call("testFun", as.integer(L))
| print(ret$X[1:5,])
| print(ret$Y[1:5,])
| 
| 
| Here's what I often get as output:
| 
| [1] 1 1 1 1 1
| [1] 1 1 1 1 1
| 
| However, the second rows should be all 2's!
| 
| When I try to pass smaller matrices, for example by setting L=100000, the
| problem goes away:
| 
| [1] 1 1 1 1 1
| [1] 2 2 2 2 2
| 
| 
| Also, the problem does not arise when I create and pass pack objects of type
| Rcpp::NumericMatrix;  so far I've see the problem only with Armadillo objects.
|  I've encountered this on two Macs running OS X 10.6.6, R 2.12.2, and I'm using
| the latest versions of Rcpp and RcppArmadillo.

I can't replicate that. I agree with you that we may have an issue here as
for 'large' N (such as 500,000) I get a segfault which is definitely
wrong (PS: but see below). On the other hand, for smaller values it works:

R> require(inline)
Loading required package: inline
R> 
R> src <- '
+ const long L  = Rcpp::as<long>(ls);
+ arma::mat X(L,1);
+ arma::mat Y(L,1);
+ X.fill(1);
+ Y.fill(2);
+ return Rcpp::List::create(Rcpp::Named("X") = Rcpp::wrap(X),
+                           Rcpp::Named("Y") = Rcpp::wrap(Y));'
R> 
R> fun <- cxxfunction(signature(ls="numeric"), body=src, plugin="RcppArmadillo")
R>                    
R> head(as.data.frame(fun(5)))
  X Y
1 1 2
2 1 2
3 1 2
4 1 2
5 1 2
R> head(as.data.frame(fun(500)))
  X Y
1 1 2
2 1 2
3 1 2
4 1 2
5 1 2
6 1 2
R> head(as.data.frame(fun(5000)))
  X Y
1 1 2
2 1 2
3 1 2
4 1 2
5 1 2
6 1 2
R> head(as.data.frame(fun(50000)))
  X Y
1 1 2
2 1 2
3 1 2
4 1 2
5 1 2
6 1 2
R> 


I quickly followed one hunch and the segfault is due to the long value for
the index. If you use an int then even my (modest) 6gb ram machine is happy to
allocate 50,000,000 objects. I stopped at that size.

R> require(inline)
Loading required package: inline
R> 
R> src <- '
+ const int L  = Rcpp::as<int>(ls);
+ arma::mat X(L,1);
+ arma::mat Y(L,1);
+ X.fill(1);
+ Y.fill(2);
+ return Rcpp::List::create(Rcpp::Named("X") = Rcpp::wrap(X),
+                           Rcpp::Named("Y") = Rcpp::wrap(Y));'
R> 
R> fun <- cxxfunction(signature(ls="numeric"), body=src, plugin="RcppArmadillo")
R> 
R> head(as.data.frame(fun(50000)))
  X Y
1 1 2
2 1 2
3 1 2
4 1 2
5 1 2
6 1 2
R> head(as.data.frame(fun(250000)))
  X Y
1 1 2
2 1 2
3 1 2
4 1 2
5 1 2
6 1 2
R> head(as.data.frame(fun(500000)))
  X Y
1 1 2
2 1 2
3 1 2
4 1 2
5 1 2
6 1 2
R> head(as.data.frame(fun(5000000)))
  X Y
1 1 1
2 1 1
3 1 1
4 1 1
5 1 1
6 1 1
R> head(as.data.frame(fun(50000000)))
  X Y
1 1 1
2 1 1
3 1 1
4 1 1
5 1 1
6 1 1
R> 

Dirk

-- 
Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com