On Jun 2, 2015, at 12:27 AM, Scott Ritchie <sritchie73 at gmail.com> wrote:
Hi Yue,
The call (float *)xpMat->matrix() is simply telling C++ to interpret the stored memory as a float, so it?s simply breaking up the stored binary data into float-sized chunks instead of double-sized chunks, so you get nonsense numbers. To store as a float, you would have to cast double_bigmat to a float type, which I believe makes a new copy of the object after casting each value, so would defeat the purpose. Also big.matrix objects in R only support char, short, int, and double as the underlying storage type (see help(?big.matrix?, ?bigmemory?)), so you wouldn?t be able to access the float matrix from R as a big.matrix (without casting it back to a double) anyway.
Regards,
Scott
On 2 June 2015 at 12:58, Yue Li <gorillayue at gmail.com <mailto:gorillayue at gmail.com>> wrote:
Dear List,
I wonder if there is a way to convert a big matrix to ?float? instead of ?double? within a Rcpp program. The reason for using float is mainly for performance improvement.
For instance, I have a simple function named ?print_bigmat? as shown below.
As shown in the output, ?double_bigmat' will save the correct values of the original matrix ?x? but not ?float_bigmat?.
// [[Rcpp::export]]
int print_bigmat(SEXP pBigMat) {
XPtr<BigMatrix> xpMat(pBigMat);
const mat& double_bigmat = arma::Mat<double>((double *)xpMat->matrix(), xpMat->nrow(), xpMat->ncol(), false);
const fmat& float_bigmat = arma::Mat<float>((float *)xpMat->matrix(), xpMat->nrow(), xpMat->ncol(), false);
Rcout << double_bigmat << endl;
Rcout << float_bigmat << endl;
return 0;
}
Output:
x <- matrix(rnorm(10), nrow=2)
x
[,1] [,2] [,3] [,4] [,5]
[1,] -0.05514382 -0.03943825 1.4145593 -0.1161918 2.3282466
[2,] -1.22023371 -0.35592125 0.7714512 0.6865120 -0.3504811
print_bigmat(as.big.matrix(x)@address)
-0.0551 -0.0394 1.4146 -0.1162 2.3282
-1.2202 -0.3559 0.7715 0.6865 -0.3505
-3.3865e-14 -8.6552e+04 4.5441e-07 -5.0912e+23 -1.5184e+34
-1.3456e+00 -1.9025e+00 -1.2828e+00 -1.6780e+00 1.9268e+00
[1] 0