[Rcpp-devel] float for BigMatrix

[Scott Ritchie]

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> 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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