[Rcpp-devel] Setting time zone on Rcpp::Datetime

[janus Larsen]

Hi again,

Thanks Dirk for your reply. So there is no way of doing this on the C side?

The thing is that I'm returning a large object that contains a lot of
information (about an unstructured 3D model) and some of the fields are
DateTimes. I could of cource wrap the call to the C function in an R
function that then corrects the dates to GMT (or sets the timezone before
calling the C function), but it just not that nice... Previously I had this
code written using <RDefines.h> where I could set the timezone on the
returned posIXct object:

SEXP long2DateTime(long dt) {
SEXP result;
result = PROTECT(allocVector(REALSXP, 1));
REAL(result)[0] = dt;
SEXP gmt = PROTECT(allocVector(STRSXP,1));
SET_STRING_ELT(gmt, 0, mkChar("GMT"));
SEXP tzone = PROTECT(allocVector(STRSXP,1));
SET_STRING_ELT(tzone, 0, mkChar("tzone"));
setAttrib(result, tzone, gmt);
SEXP datetimeclass = PROTECT(allocVector(STRSXP,2));
SET_STRING_ELT(datetimeclass, 0, mkChar("POSIXt"));
SET_STRING_ELT(datetimeclass, 1, mkChar("POSIXct"));
setAttrib(result, R_ClassSymbol, datetimeclass);
UNPROTECT(4);
return result;
}


Janus

On Fri, Jan 9, 2015 at 1:22 PM, Dirk Eddelbuettel <edd at debian.org> wrote:

>
> On 9 January 2015 at 11:50, janus Larsen wrote:
> | Hi,
> | How do I set the timezone on an Rcpp::Datetime?
> | Thanks in advance,
> | Sunaj
> |
> | This returns "1970-01-01 01:00:00 CET" (computer setting), but I want
> GMT...
>
> R does the formatting in its session based on your locale.
>
> That is generally the right thing:
>
> | // [[Rcpp::export]]
> | Rcpp::Datetime test() {
> |   double d=0;
> |   return(d);
> | }
>
> Small corrections to your code to actually export the function (under a
> safer
> name):
>
> #include <Rcpp.h>
>
> // [[Rcpp::export]]
> Rcpp::Datetime timetest(double d=0) {
>   return(d);
> }
>
> Then:
>
> R> sourceCpp("/tmp/timeQ.cpp")
> R> timetest()                       # my default is Chicago, or -5
> [1] "1969-12-31 18:00:00 CST"
> R> as.numeric(timetest())
> [1] 0
> R> Sys.setenv("TZ"="Europe/London")
> R> timetest()                       # I can select another one
> [1] "1970-01-01 01:00:00 BST"
> R> Sys.setenv("TZ"="UTC")
> R> timetest()                       # incl UTC
> [1] "1970-01-01 UTC"
> R>
> R> format(timetest(), tz="America/Chicago")
> [1] "1969-12-31 18:00:00"
> R>
>
> So you need to change the timezone _at the level of your app_ which can be
> as
> simple as writing a new Date formatter in R as per my last line.
>
> Dirk
>
> --
> http://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org
>
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