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 <gorilla...@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
> >
>
>
> Thanks much,
> Yue
>
>
>
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