In a related vein, is there any functionality - existant or planned - for "fancy" indexing (that is, selecting non-continguous submatrices) a la
X = matrix(1:24, nrow=3) Y = X[,c(2,4,7)] As it stands I'd create a new matrix and loop over the column indices to populate it. Not a big deal since my matrices aren't huge, but a touch inconvenient & I'm curious if there's a better alternative. Best, Jared On Wed, Sep 21, 2011 at 8:35 AM, Dirk Eddelbuettel <[email protected]> wrote: > > On 20 September 2011 at 23:00, Noah Silverman wrote: > | Nice suggestion, but it may not work for what I'm trying to do. > | > | I'm building up a matrix of values over a time series as part of a big loop. > | At certain iterations, I need to calculate some summary statistics on a few > | things. In R, it is trivial to subset any part of a matrix. Not sure to do > | that in Rcpp. > > RcppArmardillo makes Armadillo and Rcpp (and hence R) interoperable. > > Which means Zarrar has just solved your problem for you. I don't quite see > where that 'may not work' as you claim. Care to expand? > > You also may want to look at things like the Rcpp::SubMatrix class. Here is a > quick cut and paste from one of the unit test: > > NumericMatrix xx(4, 5); > xx(0,0) = 3; > xx(0,1) = 4; > xx(0,2) = 5; > xx(1,_) = xx(0,_); > xx(_,3) = xx(_,2); > SubMatrix<REALSXP> yy = xx( Range(0,2), Range(0,3) ) ; > NumericMatrix res = yy ; > return res; > > Supply a matrix and two ranges, get back a submatrix. So it's there. Also > note the sugar syntax used here accessing entire rows and colums, just like > in R. > > Dirk > > | > | > | -- > | Noah Silverman > | UCLA Department of Statistics > | 8117 Math Sciences Building > | Los Angeles, CA 90095 > | > | On Sep 20, 2011, at 10:09 PM, Zarrar Shehzad wrote: > | > | > | 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 > <[email protected]> > | 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 > | > | > | _______________________________________________ > | Rcpp-devel mailing list > | [email protected] > | > https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel > | > | > | > | > | > | ---------------------------------------------------------------------- > | _______________________________________________ > | Rcpp-devel mailing list > | [email protected] > | https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel > -- > New Rcpp master class for R and C++ integration is scheduled for > San Francisco (Oct 8), more details / reg.info available at > http://www.revolutionanalytics.com/products/training/public/rcpp-master-class.php > _______________________________________________ > Rcpp-devel mailing list > [email protected] > https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel > _______________________________________________ Rcpp-devel mailing list [email protected] https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel
