Sum in groups with RcppParallel - r

Question

I have written a function that desirably sums values in groups. It takes two vectors of the same length: v and g and should return a vector of length the same as unique elements in g. Groups are encoded as integers starting from zero. Using Rcpp::sourceCpp the code compiles but when called from R (sg(runif(6), rep(0:1,each = 3)) for example) returns numeric(0).
// [[Rcpp::depends(RcppParallel)]]
#include <Rcpp.h>
#include <RcppParallel.h>
using namespace Rcpp;
using namespace RcppParallel;
struct SumsInGroups: public Worker
{
const RVector<double> v;
const RVector<int> g;
RVector<double> s;
SumsInGroups(const NumericVector v, const IntegerVector g, NumericVector s): v(v), g(g), s(s) {}
SumsInGroups(const SumsInGroups& p, Split): v(p.v), g(p.g), s(p.s) {}
void operator()(std::size_t begin, std::size_t end) {
for (std::size_t i = begin; i < end; ++i) {
if (s[g[i]] != s[g[i]]) s[g[i]] = v[i];
else s[g[i]] += v[i];
}
}
void join(const SumsInGroups& rhs) {
for(std::size_t i = 0; i < s.length(); i++) {
s[i] += rhs.s[i];
}
}
};
// [[Rcpp::export]]
RVector<double> sg(NumericVector v, IntegerVector g) {
NumericVector s;
SumsInGroups p(v, g, s);
parallelReduce(0, v.length(), p);
return p.s;
}
I am very new to RcppParallel so any comments and suggestions are welcomed.

Answer 1

You need to initialize s. I suggest initializing with zeroes. Here is the code which worked for me. Note that since I initialize with zeroes, I do not need the checking you do in operator ().
#include <Rcpp.h>
using namespace Rcpp;
// [[Rcpp::depends(RcppParallel)]]
#include <RcppParallel.h>
using namespace Rcpp;
using namespace RcppParallel;
struct SumsInGroups: public Worker
{
const RVector<double> v;
const RVector<int> g;
RVector<double> s;
SumsInGroups(const NumericVector v, const IntegerVector g, NumericVector s): v(v), g(g), s(s) {}
SumsInGroups(const SumsInGroups& p, Split): v(p.v), g(p.g), s(p.s) {}
void operator()(std::size_t begin, std::size_t end) {
for (std::size_t i = begin; i < end; ++i) {
s[g[i]] += v[i];
}
}
void join(const SumsInGroups& rhs) {
for(std::size_t i = 0; i < s.length(); i++) {
s[i] += rhs.s[i];
}
}
};
// [[Rcpp::export]]
RVector<double> sg(NumericVector v, IntegerVector g) {
NumericVector s(*std::max_element(g.begin(), g.end()) + 1);
SumsInGroups p(v, g, s);
parallelReduce(0, v.length(), p);
return p.s;
}
/*** R
set.seed(101)
o <- runif(15)
i <-sample(0:3,15, rep = TRUE)
sg(o, i)
tapply(o, i, sum)
*/