On Dec 13, 2012, at 1:43 PM, Tobias Gauster wrote:
Hi,
I encountered the behavior, that the duplicated method for
data.frames gives "false positives" if there are columns of class
POSIXct with a clock shift from DST to standard time.
time <- as.POSIXct("2012-10-28 02:00", tz="Europe/Vienna") + c(0,
60*60)
time
[1] "2012-10-28 02:00:00 CEST" "2012-10-28 02:00:00 CET"
df <- data.frame(time, text="foo")
duplicated(df)
[1] FALSE TRUE
In this instance
This is because the timezone is lost after calling paste():
do.call(paste, c(df, sep = "\r"))
I suspect the problem arise when 'paste' coerces to character:
> as.character(time)
[1] "2012-10-28 02:00:00" "2012-10-28 02:00:00"
I think that as.character might get missed since the 'paste' operation
is done internally.
> as.character(time, usetz=TRUE)
[1] "2012-10-28 02:00:00 CEST" "2012-10-28 02:00:00 CET"
--
David.
[1] "2012-10-28 02:00:00\rfoo" "2012-10-28 02:00:00\rfoo"
I can't really figure out if this behavior is desired or not. If so,
a short warning in ?duplicated could be helpful. It is mentioned how
duplicated.data.frame() works, but I didn't find a hint to properly
handle POSIXct-objects.
There is no duplicated.POSIXct method
My particular problem was to cast a data.frame like this one with
cast() (which calls reshape1(), which calls duplicated()):
df2 <- data.frame(time, time1=as.numeric(time),
lab=rep(1:3, each=2), value=101:106,
text=rep(c("foo", "bar"), each=3))
library(reshape2)
Using the column of class POSIXct as a variable in the formula gives:
cast(lab*time~text, data=df2, value="value")
Aggregation requires fun.aggregate: length used as default
lab time bar foo
1 1 2012-10-28 02:00:00 0 2
2 2 2012-10-28 02:00:00 1 1
3 3 2012-10-28 02:00:00 2 0
Converting to numeric, casting and converting back works as
expected, although the timezone is not visible, because
print.data.frame() calls format.POSIXct() with, usetz = FALSE:
y <- cast(lab*time1~text, data=df2, value="value")
y$time1 <- as.POSIXct("1970-01-01 01:00") + as.numeric(y$time1)
Can anyone suggest a more elegant solution?
Best,
Tobias
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David Winsemius, MD
Alameda, CA, USA
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