[Rcpp-devel] Summing over a numeric vector.

[Cedric Ginestet]

Dear Rcpp experts,

I have the following bizarre problem. Although the second summing C++ 
function seems easier than the former, it does not produce the correct 
answer. See the following reproducible codes:

##########################################
Summing1 <- cxxfunction(signature(x="numeric"), '
     Rcpp::NumericVector Dvec(x);
     int Ne = Dvec.size();
     double out =0;
     for(int i; i<Ne; i++) out += (1/Dvec[i])/Ne;
     return Rcpp::wrap(out);
',plugin="Rcpp")
x <- rnorm(100)
Summing1(x); mean(1/x)
##########################################
gives:
[1] 0.02950222
[1] 0.02950222

##########################################
Summing2 <- cxxfunction(signature(x="numeric"), '
     Rcpp::NumericVector Dvec(x);
     int Ne = Dvec.size();
     double out =0;
     for(int i; i<Ne; i++) out += Dvec[i];
     return Rcpp::wrap(out);
',plugin="Rcpp")
x <- rnorm(100)
Summing2(x); sum(x)
############################################
gives:
[1] 0
[1] -9.220588

Any help greatly appreciated.
Cheers,


-- 
Cedric Ginestet
Centre for Neuroimaging Sciences (L3.04)
NIHR Biomedical Research Centre
Institute of Psychiatry, Box P089
Kings College London
De Crespigny Park
London
SE5 8AF
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