Converting and an arma::mat adjacency matrix into an igraph graph in C (Rcpp) - type-conversion

Question

I use Armadillo objects in some (Rcpp) code where I work with matrices.
The matrices are adjacency matrices and I need to quickly compute the components of the underlying network and though I could do this via igraph.
But I fail already at converting the adjacency matrix into something that can be used with igraph.
#include <RcppArmadillo.h>
#include <iostream>
#include <igraph-0.7.1\include\igraph.h>
using namespace arma;
// [[Rcpp::depends(RcppArmadillo)]]
// [[Rcpp::export]]
vec component_membership(const mat& adjacencymatrix) {
igraph_t g;
igraph_adjacency(&g,&adjacencymatrix,IGRAPH_ADJ_DIRECTED);
// here is more code that is immaterial to my problem
}
On compilation it complains
cannot convert 'const mat* {aka const arma::Mat<double>*}' to
'igraph_matrix_t*' for argument '2' to
'int igraph_adjacency(igraph_t*, igraph_matrix_t*, igraph_adjacency_t)'
I understand why that is the case: I believe igraph_matrix_t and arma::matrix must be fundamentally different data types. How can I convert, i.e., but how do i fix this easily?

Answer 1

As you suspected, igraph_matrix_t and arma::matrix are completely different types. The igraph documentation lists no methods that would make use of a C array for constructing an igraph_matrix_t, so I think one has to do it by hand. Something like this might work (totally untested!):
igraph_matrix_t *m;
int rc = igraph_matrix_init(m, mat.n_rows, mat.n_cols);
for (unsigned long j = 0; j < mat.n_cols; ++j)
for (unsigned long i = 0; i < mat.n_rows; ++i)
igraph_matrix_set(m, i, j, mat(i, j));

Answer 2

Following #Ralf_Stubner's suggestion I ended up using the following code. Not sure it is smart, I thought I'd share it anyways
void armamat_to_igraph_matrix(const mat &x_in, igraph_matrix_t *x_out) {
igraph_matrix_init(x_out, x_in.n_rows, x_in.n_cols);
for (unsigned long j = 0; j < x_in.n_cols; ++j)
for (unsigned long i = 0; i < x_in.n_rows; ++i)
igraph_matrix_set(x_out, i, j, x_in(i, j));
return;
}
void igraph_vector_to_armauvec(const igraph_vector_t *x_in, uvec &x_out) {
x_out = uvec(igraph_vector_size(x_in));
for (unsigned long j = 0; j < igraph_vector_size(x_in); ++j)
x_out(j) = igraph_vector_e(x_in,j);
return;
}
void igraph_vector_to_armavec(const igraph_vector_t *x_in, vec &x_out) {
x_out = vec(igraph_vector_size(x_in));
for (unsigned long j = 0; j < igraph_vector_size(x_in); ++j)
x_out(j) = igraph_vector_e(x_in,j);
return;
}