[R] Problem with POSIXct in ave

[Gabor Grothendieck]

On Fri, Aug 20, 2010 at 3:09 PM, Gabor Grothendieck
<ggrothendieck at gmail.com> wrote:
> 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)])
>

That should be (i.e. the left hand side should be revised to):
  dat$elapsed.time <- with(dat, dt - dt[match(id, id)])