How to select the row with the maximum value in ea

Question

Currently I have a problem as follows. In a dataset where multiple observations for each subject exist, and I want to make a subset of this dataset where only the maximum data for a record is selected. For example, for a data set as below:

ID <- c(1,1,1,2,2,2,2,3,3)
Value <- c(2,3,5,2,5,8,17,3,5)
Event <- c(1,1,2,1,2,1,2,2,2)

group <- data.frame(Subject=ID, pt=Value, Event=Event)

Subject 1, 2 and 3 have the biggest pt value of 5, 17 and 5 respectively. How could I first, find the biggest pt value for each subject, and then, put this observation in another data frame? This means that this subset would only have the biggest pt values for each subject.

Answer 1

do.call(rbind, lapply(split(group,as.factor(group$Subject)), function(x) {return(x[which.max(x$pt),])}))

Using Base R

Answer 2

Here's a data.table solution:

require(data.table) ## 1.9.2
group <- as.data.table(group)

If you want to keep all the entries corresponding to max values of pt within each group:

group[group[, .I[pt == max(pt)], by=Subject]$V1]
#    Subject pt Event
# 1:       1  5     2
# 2:       2 17     2
# 3:       3  5     2

If you'd like just the first max value of pt:

group[group[, .I[which.max(pt)], by=Subject]$V1]
#    Subject pt Event
# 1:       1  5     2
# 2:       2 17     2
# 3:       3  5     2

In this case, it doesn't make a difference, as there aren't multiple maximum values within any group in your data.

Answer 3

If you want the biggest pt value for a subject, you could simply use:

   pt_max = as.data.frame(aggregate(pt~Subject, group, max))

Answer 4

A dplyr solution:

library(dplyr)
ID <- c(1,1,1,2,2,2,2,3,3)
Value <- c(2,3,5,2,5,8,17,3,5)
Event <- c(1,1,2,1,2,1,2,2,2)
group <- data.frame(Subject=ID, pt=Value, Event=Event)

group %>%
    group_by(Subject) %>%
    summarize(max.pt = max(pt))

This yields the following data frame:

  Subject max.pt
1       1      5
2       2     17
3       3      5

Answer 5

A shorter solution using data.table:

setDT(group)[, .SD[which.max(pt)], by=Subject]
#    Subject pt Event
# 1:       1  5     2
# 2:       2 17     2
# 3:       3  5     2

Answer 6

The most intuitive method is to use group_by and top_n function in dplyr

    group %>% group_by(Subject) %>% top_n(1, pt)

The result you get is

    Source: local data frame [3 x 3]
    Groups: Subject [3]

      Subject    pt Event
        (dbl) (dbl) (dbl)
    1       1     5     2
    2       2    17     2
    3       3     5     2