ddply with POSIXct (vs. Date)
Joseph Voelkel
ddply seems to have difficulty with POSIXct. Here is an example (based
on my initial problem, which was using ddply to get ranges of
dates based on a grouping variable. I have omitted a grouping variable here)
try1<-structure(c(-2208988800, -2206310400, -2203891200, -2201212800,
-999475200, -996796800, -994204800, -991526400), class = c("POSIXct",
"POSIXt"), tzone = "UTC")
try1 # all whole dates
# Like try1, but 10-second change to first element
try1a<-structure(c(-2208988800-10, -2206310400, -2203891200, -2201212800,
-999475200, -996796800, -994204800, -991526400), class = c("POSIXct",
"POSIXt"), tzone = "UTC")
try1a[1:4] # 10 second change in first element...
# Like try1, but different class
try2<-structure(c(-25567, -25536, -25508, -25477, -11568, -11537, -11507,
-11476), class = "Date")
all.equal(as.Date(try1),try2)
# put into data frames for ddply use
x1<-data.frame(try1)
x1a<-data.frame(try1a)
x2<-data.frame(try2)
# base R results. These are correct
range(x1$try1)
range(x1a$try1a) # 10 second diff. 1st element is the minimum
range(x2$try2)
# But ...
ddply(x1,.(),function(dfr) range(dfr[,1])) # off by 5 or 4 hours
ddply(x1a,.(),function(dfr) range(dfr[,1])) # delta of 10 secs in min, so delta is correct
ddply(x2,.(),function(dfr) range(dfr[,1])) # OK
# as.Date "corrects" this
ddply(x1,.(),function(dfr) range(as.Date(dfr[,1])))
# and see individual returns values
(test1<-ddply(x1,.(),function(dfr) dfr[1]) )
all.equal(x1,test1[2]) # OK
# But this does not work
ddply(x1,.(),function(dfr) dfr[,1]) # off by 5 or 4 hours
Ideas?
Joe Voelkel
Hadley Wickham
try1<-structure(c(-2208988800, -2206310400, -2203891200, -2201212800,
-999475200, -996796800, -994204800, -991526400), class = c("POSIXct",
"POSIXt"), tzone = "UTC")
dlply(x1,.(),function(dfr) range(dfr[,1])) # OK
ddply(x1,.(),function(dfr) range(dfr[,1])) # NOT OK
But this sort of problem is notoriously painful in R because of how
easy it is to lose the timezone attribute.
Hadley
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Chief Scientist, RStudio
http://had.co.nz/
Joseph Voelkel
Joe