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

[Dirk Eddelbuettel]

On 19 January 2015 at 09:02, janus Larsen wrote:
| Thx - that did the trick. But why the 'Oh dear'?

Your initial code is a pretty clear exhibit of why the C API is more painful.  

Compare your current example with the one you posted first below.  "Oh dear."

Dirk

| // [[Rcpp::export]]
| Rcpp::NumericVector getDT(int dt) {
|   Rcpp::NumericVector res(1);
|   res[0] = dt;
|   res.attr("tzone") = "GMT"; 
|   res.attr("class") = CharacterVector::create("POSIXct", "POSIXt");;
|   return res;
| }
| 
| On Mon, Jan 12, 2015 at 1:34 PM, Dirk Eddelbuettel <edd at debian.org> wrote:
| 
| 
|     On 12 January 2015 at 13:10, janus Larsen wrote:
|     | 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;
|     | }
| 
|     Oh dear. See eg http://gallery.rcpp.org/articles/creating-xts-from-c++/ or
|     the other few xts related answers on the Rcpp Gallery.
|    
|     Dirk
|    
|     | 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
|     |
|     |
| 
|     --
|     http://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org
| 
| 
| 

-- 
http://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org