I am using armadillo and R through RcppArmadillo.
I have a sparse matrix and a row number as input. I would like to search the corresponding row of the matrix and return the location of the nonzeros.
So far my function looks like
// [[Rcpp::export]]
arma::uvec findAdjacentStates(arma::sp_mat adjacency, int row) {
arma::uvec out(adjacency.n_cols);
arma::SpSubview<double>::const_iterator start = adjacency.row(row).begin();
arma::SpSubview<double>::const_iterator end = adjacency.row(row).end();
for(arma::SpSubview<double>::const_iterator it = start; it != end; ++it)
{
Rcout << "location: " << it.row() << "," << it.col() << " ";
Rcout << "value: " << (*it) << std::endl;
}
return out;
}
which is based on a previous SO answer .
The function crashes R.
require(Matrix)
x = rsparsematrix(10, 10, .2)
x = x > 1
x = as(x, "dgCMatrix")
findAdjacentStates(x, 1)
One thing that is not clear to me is how to iterate on a row vector specifically; the previous SO answer was for iterating on a sparse matrix.
Update: according to valgrind the problem is in operator++ (SpSubview_iterators_meat.hpp:319), so it seems this is not the correct way to iterate on a sparse row vector
The way to iterate on a sparse matrix row is with a sp_mat::row_iterator . Unfortunately, there's no way to know ahead of time what size your output vector would be and uvec
objects don't have a push_back
like regular vectors do. Here would be my suggestion:
#include <RcppArmadillo.h>
// [[Rcpp::depends(RcppArmadillo)]]
using namespace Rcpp;
using namespace arma;
// [[Rcpp::export]]
IntegerVector findAdjacentStates(sp_mat adjacency, int row) {
IntegerVector out;
sp_mat::const_row_iterator start = adjacency.begin_row(row);
sp_mat::const_row_iterator end = adjacency.end_row(row);
for ( sp_mat::const_row_iterator i = start; i != end; ++i )
{
out.push_back(i.col());
}
return out;
}
Which we can test out easily enough:
# We need Rcpp and Matrix:
library(Rcpp)
library(Matrix)
# This is the C++ script I wrote:
sourceCpp('SOans.cpp')
# Make example data (setting seed for reproducibility):
set.seed(123)
x = rsparsematrix(10, 10, .2)
# And test the function:
findAdjacentStates(x, 1)
[1] 4
x
10 x 10 sparse Matrix of class "dgCMatrix"
[1,] . . 0.84 . 0.40 0.7 . . . -0.56
[2,] . . . . -0.47 . . . . .
[3,] . . . . . . . . -2.00 .
[4,] 0.15 . . . . . . . . -0.73
[5,] 1.80 . . . . . . . . .
[6,] . . . . . . . . 0.11 .
[7,] . . -1.10 . . . . -1.70 -1.10 .
[8,] . . . . . . . 1.30 . -0.22
[9,] -0.63 . 1.20 . . . . 0.36 . .
[10,] . . . . 0.50 -1.0 . . . .
So, we can see this works; row 1 (in C++ terms; row 2 in R terms) has only one non-zero element, which is in column 4 (in C++ terms; column 5 in R terms). This should work if you're wanting to return the output to R. If you're wanting to use the output in another C++ function, depending on what you're doing you may prefer to have a uvec
rather than an IntegerVector
, but you can just convert the IntegerVector
to a uvec
(probably not the most efficient solution, but the best I thought of right now):
// [[Rcpp::export]]
uvec findAdjacentStates(sp_mat adjacency, int row) {
IntegerVector tmp;
sp_mat::const_row_iterator start = adjacency.begin_row(row);
sp_mat::const_row_iterator end = adjacency.end_row(row);
for ( sp_mat::const_row_iterator i = start; i != end; ++i )
{
tmp.push_back(i.col());
}
uvec out = as<uvec>(tmp);
return out;
}
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