[Rcpp-devel] subsetting a matrix

[Dirk Eddelbuettel]

On 3 March 2014 at 20:13, Yixuan Qiu wrote:
| Hi Hideyoshi,
| You may try the code below:
| 
| 
| cppFunction(depends = "RcppArmadillo",
|         'NumericMatrix ss(NumericMatrix X_, IntegerVector ind_) {
|             
|              int n = X_.nrow(), k = X_.ncol();
|             
|              arma::mat X(X_.begin(), n, k, false);
|              arma::uvec ind = as<arma::uvec>(ind_);
|              arma::mat submat = X.rows(ind - 1);
|             
|              return wrap(submat);
|          }')
| 
| m = matrix(rnorm(90), ncol=3);
| ss(m, c(1, 5, 12, 22))

Nice. 

Also see 

   http://gallery.rcpp.org/articles/armadillo-subsetting/

Dirk

| 
| 
| 
| Best,
| Yixuan
| 
| 
| 2014-03-03 19:24 GMT-05:00 Hideyoshi Maeda <hideyoshi.maeda at gmail.com>:
| 
|     Hi Rcpp-devel list,
| 
|     I would like to subset a matrix by only selecting certain rows.
| 
|     Something like the below?where y is a vector of integers that represent the
|     rows to be selected. However the below does not seem to work.
| 
|     // [[Rcpp::export]]
|     Rcpp::NumericMatrix ss(SEXP X_,SEXP y_){
|       Rcpp::NumericMatrix X(X_)
|       Rcpp::NumericVector y(y_)
|       return X[y];
|     }
| 
|     R> ss(matrix(rnorm(90),ncol=3),c(1,5,12,22))
| 
|     It is a bit similar to this question http://stackoverflow.com/questions/
|     13038256/subset-of-a-rcpp-matrix-that-matches-a-logical-statement however
|     rather than just a single value, the find it for a vector.
| 
|     also I am not too sure to what extent the Armadillo submat function is able
|     to say subset for example rows: 1,5, 12 and 22 and then call the resulting
|     matrix something else?
| 
|     I was hoping that this subsetting procedure was going to be easy?but I seem
|     to have got stuck.
| 
|     thanks
| 
|     HLM
| 
| 
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| 
| 
| 
| --
| Yixuan Qiu <yixuan.qiu at cos.name>
| Department of Statistics,
| Purdue University
| 
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-- 
Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com