[Rcpp-devel] Easy way to subset a matrix

[Zarrar Shehzad]

I am not sure if there are native functions in Rcpp but you could use
RcppArmadillo to solve your problem.

So say Xs = x:

// Convert from SEXP => Rcpp => Arma
Rcpp::NumericMatrix Xr(Xs);
arma::mat X(Xr.begin(), Xr.nrow(), Xr.ncol(), false);

// Get subset of matrix and calculate variance
// (i.e., do var(x[3:10,5]))
arma::var( X.submat(3, 10, 5, 5) );

The armadillo docs are really nice so you can see those for more info on the
var and submat functions.

Cheers
Zarrar

On Wed, Sep 21, 2011 at 12:16 AM, Noah Silverman <noahsilverman at ucla.edu>wrote:

> Hello,
>
> I want to calculate the variance of a subset of a matrix column.
>
> For example, if I wanted the variance of items 3-10 in column 5.
>
> In R, this would be:
>
> x <- matrix(rnorm(100), nrow=10, ncol=10)
> varx <- var(x[3:10,5])
>
> In Rcpp, I can construct a matrix object Rcpp::NumericMatrix x
> The var function is available thanks to the great Sugar implementation
>
> But, how can I easily reference a subset of a column to calculate the
> variance?
>
> Ideas?
>
>
>   --
> Noah Silverman
> UCLA Department of Statistics
> 8117 Math Sciences Building #8208
> Los Angeles, CA 90095
>
>
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