# [R] plyr: colvar value corresponding to max Date

arun smartpink111 at yahoo.com
Thu Feb 13 17:36:42 CET 2014

```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.

On Thursday, February 13, 2014 11:15 AM, Dan Murphy <chiefmurphy at gmail.com> wrote:
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"]
 -1.007111 -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
 -1.007111 -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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