The solution I came up with myself was simply to coerce the integer back to
POSIXct:
class(ev$date) = "POSIXct"
Can't say it is the right way to do it but it seem to work.
A second related problem I haven't been able to solve as yet is how to include
"incidents" columns (those not in 'x' or 'by') in an aggregate.
names(ev): "date" "defectnum" "state"
aggregate(ev$date, by=list(ev$defectnum), max)
This returns only the date and defectnum, I also need the state.
I tried writing my own aggregator function:
maxevent = function(events) {
events[which.max(events$date),]
}
aggregate(ev, by=list(ev$defectnum), maxevent)
But I get:
Error in "[.default"(events, which.max(events$date), ) :
incorrect number of dimensions
I am trying to retrieve only the rows of ev with the latest date for a given
defectnum.
cheers
> Message: 29
> Date: Mon, 31 Jan 2005 16:16:35 +1100
> From: "Paul Sorenson" <[EMAIL PROTECTED]>
> Subject: [R] aggregating dates
> To: <r-help@stat.math.ethz.ch>
> Message-ID: <[EMAIL PROTECTED]>
> Content-Type: text/plain; charset="iso-8859-1"
>
> I have a frame which contains 3 columns:
>
> "date" "defectnum" "state"
>
> And I want to get the most recent state change for a given
> defect number. date is POSIXct.
>
> I have tried:
> aggregate(ev$date, by=list(ev$defectnum), max)
>
> Which appears to be working except that the dates seem to
> come back as integers (presumably the internal representation
> of POSIXct).
>
> When I execute max(ev$date) the result remains POSIXct.
>
> I have been dredging through the help among DateTimeClasses
> and haven't found a function that converts these integers to
> some kind of date class. Or a method for using aggregate
> which doesn't perform the conversion in the first place.
>
> Any clues?
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