> From: r-devel-bounces at r-project.org
> [mailto:r-devel-bounces at r-project.org] On Behalf Of Thomas Mang
> Sent: Monday, November 24, 2008 1:02 AM
> To: r-devel at stat.math.ethz.ch
> Subject: [Rd] timezone attribute lost
>
> Hi,
>
> As I didn't get any response on the general help list and I
> don't know
> if there is a bug in action I am trying my luck here.
>
> I was highly surprised to find out that during simple operations (see
> code below) the timezone attribute for POSIXct data is lost and then,
> upon the next interpretation, the system settings are used (which are
> plain wrong in my case).
>
> I have used R 2.8.0 under Windows XP with the system timezone
> (managed
> by Windows) set to CET - I suppose however that all other timezones,
> with the exception of GMT, will show similiar surprising
> behavior (and
> those who live in GMT-zone: If you change your timezone
> setting please
> restart R, otherwise the effect won't take place).
>
>
> # input data
> # note that the timezone is deliberately set to GMT, and of course I
> want the operations below to take place in GMT-time
> Time = as.POSIXct(strptime(c("2007-12-12 14:30:15",
"2008-04-14
> 15:31:34", "2008-04-14 15:31:34"), format = "%Y-%m-%d
%H:%M:%S", tz =
> "GMT"))
> Time # OK, time zone is GMT
> attr(Time, "tzone") # OK, time zone is GMT
>
>
> # Surprise 1:
> TApply = tapply(1:3, Time, max)
> names(TApply) # wrong, names are converted to time zone of system
>
> # Surprise 2:
> UTime = unique(Time)
> UTime # wrong, again time zone of system is used
> attr(UTime, "tzone") # == NULL
>
>
> I know how to "solve" the problem (for example by setting an R
system
> variable TZ to GMT), but I wonder why is this mess happening at all?
> Moreover, is this behavior considered to be a feature, or a
> plain bug ?
All of those problems are due to a problem in unique.default(), which
sends
the integer data in POSIXct, Date, and factor objects through a
.Internal
and then tries to reconstruct the original sort of object from the
integer output of that .Internal()
z <- .Internal(unique(x, incomparables, fromLast))
if (is.factor(x))
factor(z, levels = seq_len(nlevels(x)), labels = levels(x),
ordered = is.ordered(x))
else if (inherits(x, "POSIXct") || inherits(x, "Date"))
structure(z, class = class(x))
Your immediately problem could be solved by adding
tzone=attr(x,"tzone")
to the structure call, but I'm not familiar enough with classes
inheriting
from POSIXct and Date to know if that is sufficient. There is no reason
someone won't make a new subclass where another attribute is essential.
Since .Internal used the equivalent of as.numeric(x) to extract numeric
codes,
it might be nice to have an as.numeric<-(x,value) function that could
insert
numeric codes back into a dataset so you could avoid reconstructing an
object
of unknown structure with as.numeric(x)<-z (or perhaps as.vector<-
should
be used so you don't have to know what the integer type is). In S and
S+
one can use x at .Data<-newNumericCodes for this sort of thing, but that
can
be dangerous because it lets you stick in inappropriate types.
One might think that adding a new unique method for POSIXct or Date or
things
subclassed from them would be the right way to structure things, but
factor()
explicitly calls unique.default().
> Thanks,
> Thomas
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
Bill Dunlap
TIBCO Software Inc - Spotfire Division
wdunlap tibco.com