Dear Andrew:
Here is another way assuming you have an order column in your history data
as well as a person_id. Again, your variable
of interest is x:
# Some data
set.seed(1)
history<-data.frame(
         person_id=rep(1:10,each=10),
         record=rep(sample(10),10),
         x=rnorm(100)
        )

history

# Splitting the data by person_id
spl<-with(history,split(mydata,person_id)) # splitting by person_id
spl

# Selecting the last record of x by person_d and reporting
# the person_id and the x value  --  you can change this to your convenience
:)
do.call(rbind,lapply(spl,function(x) x[which.max(x[,2]),c(1,3)]))

HTH,

Jorge


On Fri, Feb 27, 2009 at 2:02 PM, Andrew Ziem <ahz001@gmail.com> wrote:

> I want to find the last record for each person_id in a data frame
> (from a SQL database) ordered by date.  Is there a better way than
> this for loop?
>
> for (i in 2:length(history[,1])) {
>    if (history[i, "person_id"] == history[i - 1, "person_id"])
>      history[i, "order"] = history[i - 1, "order"] + 1 # same person
>    else
>      history[i, "order"] = 1 # new person
> }
>
> # ignore all records except the last for each con_id
> history2 <- subset(history, order == 1)
>
>
> Andrew
>
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