[Rcpp-devel] strange code problem

[Carslaw, David]

Hi all

I'm new to Rcpp ... and C++, but I am keen to use it to speed up simple parts of my R package and to learn more.  I thought I had made a good start, but I have encountered a strange problem.  Now, this may well be due to my poor knowledge of C++, but I can't find where the problem is and I'd appreciate some help.

In the code below it calculates rolling means. With a simple test data set it works as I want e.g.

x <- 1:20
fun(x, 8, 75)
[1]   NA   NA   NA   NA   NA   NA   NA  4.5  5.5  6.5  7.5  8.5  9.5 10.5 11.5 12.5 13.5 14.5 15.5 16.5

And if I run it again I get the same result.

Now if I divide x by 2 and run it again I get:


x = x/2

> fun(x, 8, 75)

 [1]       NA       NA       NA       NA       NA       NA       NA 2.250000 2.531250 2.785156 3.008301

[12] 3.196838 3.346443 3.452249 3.508780 3.728627 3.940799 4.147755 4.352686 4.559667

Which is also correct.  BUT if I run the fun(x, 8, 75) again I get all NAs:


fun(x, 8, 75)

 [1] NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA

Now I appreciate that I maybe I'm doing something silly - but can you tell me what?!  This is on Windows 7, Rcpp_0.9.9.  Any help, much appreciated.

Thanks

David


library(inline)
src <- '
Rcpp::
NumericVector A(x);
double capr = as<double>(cap); // data capture %
int lenr = as<int>(len); // window size %
NumericVector res(x); // for results
LogicalVector NA(x);
NumericVector missing(1);
missing[0] = NA_REAL;
int n = A.size();
double sum = 0.0;
double sumNA = 0; // number of missings
NA = is_na(A) ; // logical vector of missings
for (int i = 0; i <= (n - lenr); i++) {
  sum = 0; // initialise
  sumNA = 0;
  for (int j = i; j < i + lenr; j++) {
    //sum += A(j); // increment sum.
    if (NA[j]) {
      sumNA += 1;
    }
    else
      {
        sum += A[j];
      }
  }

  // Calculate moving average and display.

  if (1 - sumNA / lenr < capr / 100) {
    res(i + lenr - 1) = missing[0];
  }
  else
    {
      res(i + lenr - 1) = sum / (lenr - sumNA);
    }

}

// pad out missing data
for (int i = 0; i < lenr - 1; i++) {
  res(i) = missing[0];
}
return res;
'
fun <- cxxfunction(signature(x = "numeric", len = "int", cap = "double"), body = src, plugin="Rcpp")



David Carslaw

Science Policy Group
Environmental Research Group
MRC-HPA Centre for Environment and Health
King's College London
Room 4.129 Franklin Wilkins Building
Stamford Street
London SE1 9NH
david.carslaw at kcl.ac.uk<mailto:david.carslaw at kcl.ac.uk>
T 020 7848 3844

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