[Rcpp-devel] Filling a big.matrix in Rcpp

[Shraddha Pai]

Hi all,
Thanks for all the responses and for clarifying what big matrices are.

I had actually looked through the rcpp-devel site before attempting to use
big.matrix with Rcpp. I probably misunderstood this thread about
bigmemory; it sounded like bigmemory objects worked "out-of-the-box" with
Rcpp (and maybe Xptr could play a role).
http://r.789695.n4.nabble.com/looking-for-adice-on-bigmemory-framework-with
-C-and-java-interoperability-td4610333.html

I presume the below is consistent with Michael's earlier post suggesting
use of a BigMatrix pointer, and that the old thread was just discussed the
data structure of big.matrix objects (rather than that they can be used as
"just doubles").


>> bigmemory matrices are simply arrays of native types (typically
>>doubles, 
>> but bm supports other types, too) so they are trivially
>>readable/writable from both
>>C++ (just read into memory and cast to the array type)

I'll try the code posted below and confirm.

Regards,
Shraddha


On 13-03-14 8:57 PM, "Dirk Eddelbuettel" <edd at debian.org> wrote:

>
>On 15 March 2013 at 00:51, Romain Francois wrote:
>| Why do you use wrap here ?
>
>Reflex, because I returned an atomistic type.
>
>| Wrap will create a SEXP, not a bool !
>
>Quite right but was in a rush while doing other things too. "Forgot" that
>I
>was using sourceCpp() which give one additional layer of indirection.
> 
>| Actually why is this not void ?
>
>Quite right.  Better version below. Thanks!
>
>Ultimately, we do of course want a version that takes an XPtr to a
>big.matrix
>and does something sensible and returns it.
>
>Dirk
>
>
>// -*- mode: C++; c-indent-level: 4; c-basic-offset: 4; indent-tabs-mode:
>nil; -*-
>
>#include <Rcpp.h>
>
>// The next line is all it takes to find the bigmemory
>// headers -- thanks to the magic of Rcpp attributes
>
>// [[Rcpp::depends(bigmemory)]]
>#include <bigmemory/MatrixAccessor.hpp>
>
>
>// We define a simple function, and pass the incoming XPtr as a SEXP;
>// we could also pass a templated XPtr. Function returns only a bool.
>
>// [[Rcpp::export]]
>void fun(SEXP A) {
>    Rcpp::XPtr<BigMatrix> bigMat(A);
>    MatrixAccessor<int> Am(*bigMat);
>    
>    int nrows = bigMat->nrow();
>    int ncolumns = bigMat->ncol();
>    for (int j = 0; j < ncolumns; j++){
>	for (int i = 1; i < nrows; i++){
>            Am[j][i] = Am[j][i] + Am[j][i-1];
>        }
>    }
>    return;
>}
>
>// R code for testing below
>
>/*** R
>require(bigmemory)
>
># set up big.matrix
>nrows <- 10000
>bkFile <- "bigmat.bk"
>descFile <- "bigmatk.desc"
>bigmat <- filebacked.big.matrix(nrow=nrows, ncol=3,type="integer", init=1,
>				backingfile=bkFile, backingpath=".",descriptorfile=descFile,
>				dimnames=c(NULL,NULL))
>matDesc <- bigmemory::describe(bigmat)
>
>fun(bigmat at address)
>
>cat("Done\n")
>*/
>
>
>-- 
>Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com
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