# Armadillo subsetting

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A StackOverflow question asked how convert from `arma::umat`

to `arma::mat`

. The former is format used for `find`

and other logical indexing.

For the particular example at hand, a call to the `conv_to`

converter provided the solution. We rewrite the answer here using the newer format offered by Rcpp attributes and its `sourceCpp()`

function.

#include <RcppArmadillo.h> // [[Rcpp::depends(RcppArmadillo)]] using namespace Rcpp ; // [[Rcpp::export]] arma::mat matrixSubset(arma::mat M) { // logical conditionL where is transpose larger? arma::umat a = trans(M) > M; arma::mat N = arma::conv_to<arma::mat>::from(a); return N; } M <- matrix(1:9, 3, 3) M [,1] [,2] [,3] [1,] 1 4 7 [2,] 2 5 8 [3,] 3 6 9 matrixSubset(M) [,1] [,2] [,3] [1,] 0 0 0 [2,] 1 0 0 [3,] 1 1 0

This generalizes to other uses, and the vector or matrix of unsigned ints can be used inside the `elem()`

member function. Here were we return all values of `M * M'`

that are greater or equal to 100.

// [[Rcpp::export]] arma::vec matrixSubset2(arma::mat M) { arma::mat Z = M * M.t(); arma::vec v = Z.elem( arma::find( Z >= 100 ) ); return v; }

The result:

matrixSubset2(M) [,1] [1,] 108 [2,] 108 [3,] 126

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