[Rcpp-devel] bizarre behavior of RcppMatrix

[Yi-Hsin Erica Tsai]

Hi,

I've just started using Rcpp with the hopes of speeding up some R loops 
through replacement with C++ code.  I've been struggling with 
understanding the behavior of the NumericMatrix datatype and more 
generally how to pass matrices between R and C++.

I seem to be getting bizarre behavior when I pass default parameters to 
my C++ functions.  For example, take this C++ function and the 
corresponding R wrapper function:

______________________________________

//testRcppMatrix.cpp

#include <Rcpp.h>

RcppExport SEXP plusonediag_RcppMatrix(SEXP matrix)
{
     Rcpp::NumericMatrix orig(matrix);
     Rcpp::NumericMatrix plus(matrix);

	int n = orig.rows();
	int k = orig.cols();

	for (int i=0; i<n; i++)
	{
		if(i <= k)
		{
			plus(i,i) = plus(i,i) + 1;
		}
     }

     Rcpp::Pairlist res(Rcpp::Named( "original", orig),
     	Rcpp::Named( "plus", plus)
         );

     return res;
}
______________________________________

#testRcpp.r

plusonediag_RcppMatrix_wrapper <- function(mat=matrix(seq(1,9)^2, ncol=3))
{
     ## Make the call...
     val <- .Call("plusonediag_RcppMatrix",
                  mat)
     return(val)
}

______________________________________

If I pass actual values to the plusonediag_RcppMatrix_wrapper, then I 
get correct output; however, if I try to use the default parameter (mat) 
or pass a matrix equal to the default matrix, then the original matrix 
appears to be overwritten.  See the session output below.

Anyone have any idea what I'm doing wrong?  I'd appreciate any help or 
insight into this problem.

I've attached the test files if that's helpful.

Thanks and best wishes,
--Erica Tsai

 > # main.r
 >
 > library(Rcpp)
Loading required package: inline
 > dyn.load("testRcppMatrix.so")
 > source("testRcppMatrix.r")
 >
 > foo <- matrix(1:9, 3, 3)
 > bar <- foo ^ 2
 >
 > plusonediag_RcppMatrix_wrapper #default mat is same as bar
function(mat=matrix(seq(1,9)^2, ncol=3))
{
     ## Make the call...
     val <- .Call("plusonediag_RcppMatrix",
                  mat)
     return(val)
}
 > plusonediag_RcppMatrix_wrapper() #does not produce correct output!!
$original
      [,1] [,2] [,3]
[1,]    2   16   49
[2,]    4   26   64
[3,]    9   36   82

$plus
      [,1] [,2] [,3]
[1,]    2   16   49
[2,]    4   26   64
[3,]    9   36   82

 > plusonediag_RcppMatrix_wrapper(foo)  #produces correct output
$original
      [,1] [,2] [,3]
[1,]    1    4    7
[2,]    2    5    8
[3,]    3    6    9

$plus
      [,1] [,2] [,3]
[1,]    2    4    7
[2,]    2    6    8
[3,]    3    6   10

 > plusonediag_RcppMatrix_wrapper(bar)  #does not produce correct output!!
$original
      [,1] [,2] [,3]
[1,]    2   16   49
[2,]    4   26   64
[3,]    9   36   82

$plus
      [,1] [,2] [,3]
[1,]    2   16   49
[2,]    4   26   64
[3,]    9   36   82

 > plusonediag_RcppMatrix_wrapper(matrix((1:9), 3, 3))  #produces 
correct output
$original
      [,1] [,2] [,3]
[1,]    1    4    7
[2,]    2    5    8
[3,]    3    6    9

$plus
      [,1] [,2] [,3]
[1,]    2    4    7
[2,]    2    6    8
[3,]    3    6   10

 > plusonediag_RcppMatrix_wrapper(matrix((1:9)^2, 3, 3))  #does not 
produce correct output!!
$original
      [,1] [,2] [,3]
[1,]    2   16   49
[2,]    4   26   64
[3,]    9   36   82

$plus
      [,1] [,2] [,3]
[1,]    2   16   49
[2,]    4   26   64
[3,]    9   36   82

 >

-- 
Yi-Hsin Erica Tsai, Ph.D.
NSF Postdoctoral Fellow
Department of Biological Sciences (Carstens lab)
Louisiana State University

http://www.duke.edu/~yet2/
etsai at lsu.edu
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