# [Rcpp-devel] RcppEigen: Avoiding accessing elements with coeff(i, j) in sparse matrix

Douglas Bates bates at stat.wisc.edu
Tue Mar 25 16:51:06 CET 2014

```Two other small points.  I am assuming that the set of indices in the
second argument is sorted.  Also, it would be best to declare the function
as

bool foo(const MSpMat X, const Eigen::Map<Eigen::VectorXi> idx) as in the
enclosed

On Tue, Mar 25, 2014 at 10:45 AM, Douglas Bates <bates at stat.wisc.edu> wrote:

> The enclosed works on this example.  The logic is to check each column in
> the index set for all the elements of the index set, except the one on the
> diagonal, being in the set of row indices. I have added some diagnostic
> output to demonstrate the flow.
>
>
>
>
> On Tue, Mar 25, 2014 at 9:48 AM, Douglas Bates <bates at stat.wisc.edu>wrote:
>
>> On Tue, Mar 25, 2014 at 6:42 AM, Søren Højsgaard <sorenh at math.aau.dk>wrote:
>>
>>> Dear all,
>>> I have a large sparse adjacency matrix X for an undirected graph. For a
>>> subset 'idx' of the vertices I want to find out if the subgraph defined by
>>> this subset is complete (i.e. has edges between all variables). So in R,
>>> one would check if X[idx,idx] has only ones outside the diagonal, but this
>>> is slow and I want to do it with Rcpp. Here is one attempt where I simply
>>> add up the elements above (or below) the diagonal, and to access the
>>> elements of X I use coeff which is relatively slow (because X is a sparse
>>> matrix).
>>>
>>> #include <RcppEigen.h>
>>> typedef Eigen::MappedSparseMatrix<double> MSpMat;
>>> typedef Eigen::SparseVector<double> SpVec;
>>> typedef SpVec::InnerIterator InIter;
>>>
>>> //[[Rcpp::export]]
>>> double foo (MSpMat X, Eigen::VectorXi idx){
>>>   SpVec sidx = idx.sparseView();
>>>   double out=0;
>>>   int i2, j2;
>>>   for (InIter i_(sidx); i_; ++i_){
>>>     i2 = i_.index();
>>>     for (InIter j_(sidx); j_; ++j_){
>>>       j2 = j_.index();
>>>       if (i2>j2)
>>>         out += X.coeff( i2, j2);
>>>     }
>>>   }
>>>   return out;
>>> }
>>>
>>> /*** R
>>> library(Matrix)
>>> M1 <- new("dgCMatrix"
>>>     , i = c(1L, 2L, 3L, 0L, 2L, 3L, 0L, 1L, 3L, 0L, 1L, 2L, 4L, 5L, 3L,
>>> 5L, 3L, 4L)
>>>     , p = c(0L, 3L, 6L, 9L, 14L, 16L, 18L)
>>>     , Dim = c(6L, 6L)
>>>     , Dimnames = list(c("a", "b", "c", "d", "e", "f"), c("a", "b", "c",
>>> "d", "e", "f"))
>>>     , x = c(1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1)
>>>     , factors = list()
>>> )
>>> M1
>>>
>>> foo(M1, c(2,3,4))
>>>
>>> */
>>> Can anyone see a better way of doing this?
>>>
>>> I was thinking about whether the sparse matrix iterators could be a
>>> solution, but I can't a hold on it.
>>
>>
>> Sparse matrix inner and outer iterators are definitely the way to go.
>>  I'll write a version using them.
>>
>>
>>> I was also thinking about using sparse matrices in RcppArmadillo, but I
>>> guess the problem (speed) is the same (haven't tried!).
>>>
>>
>>
>>> Any advice will be greatly appreciated.
>>> Cheers
>>> Søren
>>>
>>>
>>>
>>> _______________________________________________
>>> Rcpp-devel mailing list
>>> Rcpp-devel at lists.r-forge.r-project.org
>>> https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel
>>>
>>>
>
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```