Hi,
I am having trouble using the ave function with a POSIXct object. For
example:
x<-Sys.time()+0:9*3600
dat<-data.frame(id=rep(c('a','
b','c'),each=10),dt=rep(x,3),i=rep(1:10,3))
dat
# This is what I want to do:
dat$time.elapsed<-unsplit(lapply(split(dat$dt,dat$id),function(x)
x-x[1]),f=dat$id)
dat
# The above code does the trick, but from the standpoint of simplicity, the
ave function seems to be perfect for this problem:
ave(dat$dt,dat$id,FUN=function(x) difftime(x,x[1]))
# Above doesn't work, returns: Error in as.POSIXct.default(value) : do not
know how to convert 'value' to class "POSIXct"
# Same error message with this:
ave(dat$dt,dat$id,FUN=function(x) x - x[1])
# But the following (using numeric data instead of POSIXct) does work:
ave(dat$i,dat$id,FUN=function(x) x-x[1])
# Problem seems to be in an assignment statement with split on the lhs
split(dat$dt,dat$id)<-lapply(split(dat$dt,dat$id),function(x) x-x[1])
# Because rhs works:
lapply(split(dat$dt,dat$id),function(x) x-x[1])
# And this works without assignment
split(dat$dt,dat$id)
Does anyone have any ideas about how to get ave to work with POSIXct
objects, or the cause of this (apparent) problem? I am using R v. 2.11.1.
Thanks,
Sasha
[[alternative HTML version deleted]]
On Aug 20, 2010, at 10:56 AM, Sasha Hafner wrote:> Hi, > > I am having trouble using the ave function with a POSIXct object. For > example: > > x<-Sys.time()+0:9*3600 > dat<-data.frame(id=rep(c('a',' > b','c'),each=10),dt=rep(x,3),i=rep(1:10,3)) > dat > > # This is what I want to do: > dat$time.elapsed<-unsplit(lapply(split(dat$dt,dat$id),function(x) > x-x[1]),f=dat$id) > dat > > # The above code does the trick, but from the standpoint of > simplicity, the > ave function seems to be perfect for this problem: > ave(dat$dt,dat$id,FUN=function(x) difftime(x,x[1]))Try: dat$elapsed <- ave(dat$dt,dat$id,FUN=function(x) as.POSIXct(as.numeric(x,x[1]), origin="1970-01-01")) dat I don't know why difftime doesn't work, but the above uses the "naked" numeric time in seconds and then restores the DateTime atrributes. -- David.> # Above doesn't work, returns: Error in as.POSIXct.default(value) : > do not > know how to convert 'value' to class "POSIXct" > > # Same error message with this: > ave(dat$dt,dat$id,FUN=function(x) x - x[1]) > > # But the following (using numeric data instead of POSIXct) does work: > ave(dat$i,dat$id,FUN=function(x) x-x[1]) > > # Problem seems to be in an assignment statement with split on the lhs > split(dat$dt,dat$id)<-lapply(split(dat$dt,dat$id),function(x) x-x[1]) > > # Because rhs works: > lapply(split(dat$dt,dat$id),function(x) x-x[1]) > > # And this works without assignment > split(dat$dt,dat$id) > > Does anyone have any ideas about how to get ave to work with POSIXct > objects, or the cause of this (apparent) problem? I am using R v. > 2.11.1. > > Thanks, > Sasha > > [[alternative HTML version deleted]] > > ______________________________________________ > R-help at r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code.David Winsemius, MD West Hartford, CT
On Fri, Aug 20, 2010 at 10:56 AM, Sasha Hafner <sdhafner at gmail.com> wrote:> Hi, > > I am having trouble using the ave function with a POSIXct object. For > example: > > x<-Sys.time()+0:9*3600 > dat<-data.frame(id=rep(c('a',' > b','c'),each=10),dt=rep(x,3),i=rep(1:10,3)) > dat > > # This is what I want to do: > dat$time.elapsed<-unsplit(lapply(split(dat$dt,dat$id),function(x) > x-x[1]),f=dat$id) > dat > > # The above code does the trick, but from the standpoint of simplicity, the > ave function seems to be perfect for this problem: > ave(dat$dt,dat$id,FUN=function(x) difftime(x,x[1])) > # Above doesn't work, returns: Error in as.POSIXct.default(value) : do not > know how to convert 'value' to class "POSIXct" > > # Same error message with this: > ave(dat$dt,dat$id,FUN=function(x) x - x[1]) > > # But the following (using numeric data instead of POSIXct) does work: > ave(dat$i,dat$id,FUN=function(x) x-x[1]) > > # Problem seems to be in an assignment statement with split on the lhs > split(dat$dt,dat$id)<-lapply(split(dat$dt,dat$id),function(x) x-x[1]) > > # Because rhs works: > lapply(split(dat$dt,dat$id),function(x) x-x[1]) > > # And this works without assignment > split(dat$dt,dat$id) > > Does anyone have any ideas about how to get ave to work with POSIXct > objects, or the cause of this (apparent) problem? I am using R v. 2.11.1. >If dat is sorted by id and dt (as is the case in your example) then try: dat$dt <- with(dat, dt - dt[match(id, id)])