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

[Danas Zuokas]

Yes it is the same question on SO and I did consider RHertel's comments.
But this problem (sums by group id) is not parallelFor it is parallelReduce:
I split vector, calculate sums and then aggregate those sums.
Please correct me if I am wrong.

2015-07-14 13:54 GMT+03:00 Dirk Eddelbuettel <edd at debian.org>:

>
> 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.
> |
> | _______________________________________________
> | 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
>
> --
> http://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.r-forge.r-project.org/pipermail/rcpp-devel/attachments/20150714/1c38afbe/attachment.html>