Passing a `data.table` to c++ functions using `Rcpp` and/or `RcppArmadillo`

Question

Is there a way to pass a data.table objects to c++ functions using Rcpp and/or RcppArmadillo without manually transforming to data.table to a data.frame? In the example below test_rcpp(X2) and test_arma(X2) both fail with c++ exception (unknown reason).

R code

X=data.frame(c(1:100),c(1:100))
X2=data.table(X)
test_rcpp(X)
test_rcpp(X2)
test_arma(X)
test_arma(X2)

c++ functions

NumericMatrix test_rcpp(NumericMatrix X) {
    return(X);
}

mat test_arma(mat X) {
    return(X);
}

Answer 1

Building on top of other answers, here is some example code:

#include <Rcpp.h>
using namespace Rcpp ;

// [[Rcpp::export]]
double do_stuff_with_a_data_table(DataFrame df){
    CharacterVector x = df["x"] ;
    NumericVector   y = df["y"] ;
    IntegerVector   z = df["v"] ;

    /* do whatever with x, y, v */
    double res = sum(y) ;
    return res ;
}

So, as Matthew says, this treats the data.table as a data.frame (aka a Rcpp::DataFrame in Rcpp).

require(data.table)
DT <- data.table(
    x=rep(c("a","b","c"),each=3), 
    y=c(1,3,6), 
    v=1:9)
do_stuff_with_a_data_table( DT ) 
# [1] 30

This completely ignores the internals of the data.table.

Answer 2

Try passing the data.table as a DataFrame rather than NumericMatrix. It is a data.frame anyway, with the same structure, so you shouldn't need to convert it.

Answer 3

Rcpp sits on top of native R types encoded as SEXP. This includes eg data.frame or matrix.

data.table is not native, it is an add-on. So someone who wants this (you?) has to write a converter, or provide funding for someone else to write one.

Answer 4

For reference, I think the good thing is to output a list from rcpp as data.table allow update via lists.

Here is a dummy example:

cCode <- 
    '
     DataFrame DT(DTi);
     NumericVector x = DT["x"];
     int N = x.size();
     LogicalVector b(N);
     NumericVector d(N);
     for(int i=0; i<N; i++){
         b[i] = x[i]<=4;
         d[i] = x[i]+1.;
     }
     return Rcpp::List::create(Rcpp::Named("b") = b, Rcpp::Named("d") = d);
    ';

require("data.table");
require("rcpp");
require("inline");
DT <- data.table(x=1:9,y=sample(letters,9)) #declare a data.table
modDataTable <- cxxfunction(signature(DTi="data.frame"), plugin="Rcpp", body=cCode)

DT_add <- modDataTable(DT)  #here we get the list
DT[, names(DT_add):=DT_add] #here we update by reference the data.table