[Rcpp-devel] When calling same Rcpp function several times different results are returned

[Dirk Eddelbuettel]

On 14 July 2015 at 09:25, Danas Zuokas wrote:
| I have written parallel implementation of sums in groups using RcppParallel.

Isn't this the same question as
http://stackoverflow.com/questions/31318419/when-calling-same-rcpp-function-several-times-different-results-are-returned

You got some excellent comments there by SO user 'RHertel'. Did you consider those?

Dirk
 
| // [[Rcpp::depends(RcppParallel)]]
| #include <Rcpp.h>
| #include <RcppParallel.h>
| using namespace Rcpp;
| using namespace RcppParallel;
| 
| struct SumsG: public Worker
| {
|   const RVector<double> v;
|   const RVector<int> gi;
| 
|   RVector<double> sg;
| 
|   SumsG(const NumericVector v, const IntegerVector gi, NumericVector sg): v(v), gi(gi), sg(sg) {}
|   SumsG(const SumsG& p, Split): v(p.v), gi(p.gi), sg(p.sg) {}
| 
|   void operator()(std::size_t begin, std::size_t end) {
|     for (std::size_t i = begin; i < end; i++) {
|       sg[gi[i]] += v[i];
|     }
|   }
| 
|   void join(const SumsG& p) {
|     for(std::size_t i = 0; i < sg.length(); i++) {
|       sg[i] += p.sg[i];
|     }
|   }
| };
| 
| // [[Rcpp::export]]
| List sumsingroups(NumericVector v, IntegerVector gi, int ni) {
|   NumericVector sg(ni);
|   SumsG p(v, gi, sg);
|   parallelReduce(0, v.length(), p);
| 
|   return List::create(_["sg"] = p.sg);
| }
| 
| It compiles using Rcpp::sourceCpp. Now when I call it from R sumsingroups(1:10,
| rep(0:1, each = 5), 2) several times I get the right answer (15 40) and then
| something different (usually some multiplicative of the right answer). Running
| 
| 
| res <- sumsingroups(1:10, rep(0:1, each = 5), 2)
| for(i in 1:1000) {
|     tmp <- sumsingroups(1:10, rep(0:1, each = 5), 2)
|     if(res[[1]][1] != tmp[[1]][1]) break
|     Sys.sleep(0.1)
| }
| 
| breaks at random iteration returning
| 
| $sg
| [1]  60 160
| 
| or
| 
| $sg
| [1] 30 80
| 
| I am new to Rcpp and RcppParallel and do not know what could cause such
| behavior.
| 
| Things that did not help:
| 
|  1. Added for (std::size_t i = 0; i < sg.length(); i++) sg[i] = 0; to both of
|     constructors.
|  2. Changed names so that they are different in Worker definition and in
|     function implementation.
| 
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