[R] Problem with POSIXct in ave

[David Winsemius]

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]]
>
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David Winsemius, MD
West Hartford, CT