[R] selecting rows by maximum value of one variables in dataframe nested by another Variable

[Rui Barradas]

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
>> --
>>
>> Jetzt informieren: http://mobile.1und1.de/?ac=OM.PW.PW003K20328T7073a
>>
>> ______________________________________________
>> R-help at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>