Calling igraph from within Rcpp - r

Question

As a part of utilizing network data drawn at random before further processing, I am trying to call a couple of functions from the igraph package at the beginning of each iteration. The code I use is as follows:
#define ARMA_64BIT_WORD
#include <RcppArmadillo.h>
// [[Rcpp::depends(RcppArmadillo)]]
// [[Rcpp::plugins(cpp11)]]
using namespace Rcpp;
using arma::sp_mat;
// [[Rcpp::export]]
sp_mat adj_mat(int n, double p) {
Environment igraph("package:igraph");
Function game_er = igraph["erdos.renyi.game"];
Function get_adjacency = igraph["get.adjacency"];
List g = game_er(Named("n", n), Named("p", p));
NumericMatrix A_m = get_adjacency(Named("g", g));
sp_mat A = as<sp_mat>(A_m);
return A;
}
/*** R
set.seed(20130810)
library(igraph)
adj_mat(100, 0.5)
*/
So, while the C++ compiles without warnings, the following error is thrown:
> sourceCpp("Hooking-R-in-cpp.cpp")
> set.seed(20130810)
> library(igraph)
> adj_mat(100, 0.5)
Error in adj_mat(100, 0.5) :
Not compatible with requested type: [type=S4; target=double].
From the error it looks like I am passing a S4 class to a double? Where is the error?

Answer 1

You were imposing types in the middle of your C++ functions that did not correspond to the representation, so you got run-time errors trying to instantiate them.
The version below works. I don't know igraph well enough to suggest what else you use to store the first return; for the S4 you can use the dgCMatrix matrix but S4 is an ok superset.
#include <RcppArmadillo.h>
// [[Rcpp::depends(RcppArmadillo)]]
// [[Rcpp::plugins(cpp11)]]
using namespace Rcpp;
using arma::sp_mat;
// [[Rcpp::export]]
sp_mat adj_mat(int n, double p) {
Environment igraph("package:igraph");
Function game_er = igraph["erdos.renyi.game"];
Function get_adjacency = igraph["get.adjacency"];
SEXP g = game_er(Named("n", n), Named("p", p));
S4 A_m = get_adjacency(Named("g", g));
sp_mat A = as<sp_mat>(A_m);
return A;
}
/*** R
set.seed(20130810)
library(igraph)
adj_mat(100, 0.5)
*/