Grokbase Groups R r-help June 2012
FAQ
Hello,

Here's a solution using aggregate and merge. I've kept it in two steps
for clarity.



d <- read.table(text="
subject time.ms V3
1 1 stringA
1 12 stringB
1 22 stringC
2 1 stringB
2 14 stringC
2 25 stringA
", header=TRUE)

ag <- aggregate(time.ms~subject, data=d, max)
merge(ag, d)

# It also works if the maximum is not unique
d2 <- rbind(d, c(1, 22, "stringA"))
ag2 <- aggregate(time.ms~subject, data=d2, max)
merge(ag2, d2)


The split version would have to be slightly modified, to make use of
'which' and 'max' separately.


do.call("rbind",lapply(split(d2, d2$subject), function(x)
x[which(x[, 2] == max(x[, 2])), ]))

Hope this helps,

Rui Barradas

Em 27-06-2012 09:30, Petr PIKAL escreveu:
Hi
How could I select the rows of a dataset that have the maximum value in
one variable and to do this nested in another variable. It is a dataframe
in long format with repeated measures per subject.
I was not successful using aggregate, because one of the columns has
You could do it by aggregate and subsequent selection matching values from
your data frame but it is perfect example for powerfull list operations
do.call("rbind",lapply(split(test, test$subject), function(x)
x[which.max(x[,2]),]))
subject time.ms V3
1 1 22 stringC
2 2 25 stringA
split splits data frame test according to subject variable into list of
sub data frames
function x computes which is maximum value in second column in each sub
data frame and selects the appropriate row
do.call takes the list and rbinds it to one final data frame.

Regards
Petr
character values (and/or possibly because of another reason).
I would like to transfer something like this:
subject time.ms V3
1 1 stringA
1 12 stringB
1 22 stringC
2 1 stringB
2 14 stringC
2 25 stringA
?.
To something like this:
subject time.ms V3
1 22 stringC
2 25 stringA
?

Thank you very much for you help!
Miriam
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