Hi Andrew,
Just remember that something was bad with the code I sent you (the same you
referred to in [1]).

This version runs with no problems:

# Some data
history_ <-
data.frame(person_id=c(1,2,2),date_=c("2009-01-01","2009-02-03","2009-02-02"),x=c(0.01,0.05,0.06))
colnames(history_) <- c("person_id", "date_","x")
history_

# What you needed
spl<-with(history_,split(history_,person_id))
do.call(rbind,lapply(spl,function(x) x[which.max(x[,2]),c(1,3)]))

HTH,

Jorge


On Fri, Feb 27, 2009 at 5:47 PM, Andrew Ziem <ahz001@gmail.com> wrote:

> On Fri, Feb 27, 2009 at 2:10 PM, William Dunlap <wdunlap@tibco.com> wrote:
> > Andrew, it makes it easier to help if you supply a typical
> > input and expected output along with your code.  I tried
> > your code with the following input:
>
> I'll be careful to avoid these mistakes.  Also, I should not have used
> a reserved word for the variable history, and I should have mentioned
> the data is sorted with the most recent dates first. Talk about a bad
> day! :)
>
> Originally I omitted this code before the for loop:
>
> history["order"] <- NA
> history[1,"order"] = 1
>
> Here's a sample data set:
> history_ <-
> data.frame(person_id=list(c(1,2,2)),date_=list(c("2009-01-01","2009-02-03","2009-02-02")),
> x=list(c(0.01,0.05,0.06)) )
> colnames(history_) <- c("person_id", "date_","x")
> history_
>
> Jorge's suggestion[1] works for me, and it seems much faster.  I
> simply adapted it by replacing Jorge's variable x with a sequential
> identifier already in the database.
> [1] https://stat.ethz.ch/pipermail/r-help/2009-February/189981.html
>
> > The following function, f2, does what I think you are saying
> > you want.  It sorts the data by person_id, breaking ties with
> > date, and then selects the rows where the person_id entry does
>
> My data is already sorted by the SQL database like this
>  ORDER BY person_id, date_ DESC
>
> Thanks everyone for responding and expanding my knowledge of R!
>
>
> Best regards,
> Andrew
>
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