r - Sum in groups with RcppParallel -

i have written function desirably sums values in groups. takes 2 vectors of same length: v , g , should return vector of length same unique elements in g. groups encoded integers starting zero. using rcpp::sourcecpp code compiles when called r (sg(runif(6), rep(0:1,each = 3)) 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) {     (std::size_t = begin; < 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 = 0; < 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 new rcppparallel comments , suggestions welcomed.

you need initialize s. suggest initializing zeroes. here code worked me. note since initialize zeroes, not need checking 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) {         (std::size_t = begin; < end; ++i) {             s[g[i]] += v[i];         }     }      void join(const sumsingroups& rhs) {         for(std::size_t = 0; < 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) <-sample(0:3,15, rep = true) sg(o, i) tapply(o, i, sum) */