plyr: colvar value corresponding to max Date

Hi,
Try ?which.max() # unique values for the combination.

?ddply(data,.(state),summarize,max_date=value[which.max(date)])[,2]
#or
?ddply(data,.(state),summarize,max_date=value[date == max(date)])[,2]


A.K.

I can do this in multiple steps with summarise, joins, etc., but can't
help thinking it can be accomplished in one plyr call.

Here's a small example:

require(plyr)
require(lubridate)
data <- data.frame(

+?  date = rep(as.Date(ymd(20140101 + (0:3) * 100)), 2),
+?  state = rep(c("A", "B"), each = 4),
+?  value = rnorm(8))

What I want is clearly

data[c(4, 8), "value"]

[1] -1.007111 -1.527541

Here are my multiple steps:

statemaxval <- ddply(data, "state", summarise, max_date = max(date))
rslt <- join(data, statemaxval, by = "state")
rslt <- subset(rslt, date == max_date)
rslt <- rslt[!duplicated(rslt), ]
rslt$value

[1] -1.007111 -1.527541

Is there a one-step way to accomplish this?
Something like
ddply(data, "state", summarise, "GiveMeTheValueCorrespondingToMaxDateByState!!")

Or is that only possible if there is only one unique value for a given
combination of state and max(date)?

Thanks,
Dan

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