Using RcppArmadillo each_col with lambda function? - r

Question

According to the Armadillo website, you can pass in a lambda function into .each_col, such as
X.each_col( [](vec& a){ a.print(); } );
The following Rcpp seems to have an error though, reporting "Expected Expression"
#include <RcppArmadillo.h>
// [[Rcpp::depends(RcppArmadillo)]]
using namespace Rcpp;
using namespace arma;
// [[Rcpp::export]]
arma::vec colCumSum(const arma::mat& X) {
return X.each_col( [](const arma::vec& b){ b.cumsum(); } );
}

Answer 1

You actually have to tell R to use C++11 in order to have lambda support. The magic line is [[Rcpp::plugins("cpp11")]] which makes it all work:
But once I do that I get issues on the cumsum(). You also had too many const in there.
So here is a simpler version which does work with another lambda from the documentation -- which just prints. I also turned to ivec and imat for consistency:
#include <RcppArmadillo.h>
// [[Rcpp::depends(RcppArmadillo)]]
// [[Rcpp::plugins("cpp11")]]
// [[Rcpp::export]]
arma::ivec colCumSum(arma::imat& X) {
X.each_col( [](arma::ivec& a){ a.print(); } );
return X.col(0);
}
/*** R
M <- matrix(1:16, 4, 4)
colCumSum(M)
*/
When you source this, it builds and runs. You will need to work out the lambda use case for the reduction that cumsum() does.
> sourceCpp("/tmp/foo.cpp")
> M <- matrix(1:16, 4, 4)
> colCumSum(M)
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[,1]
[1,] 1
[2,] 2
[3,] 3
[4,] 4
>