Formatting the output in R

Question

I have a set of data which shows the visit ID and the subject name

visit<-c(1,2,3,1,2,1,1,2,3,1,2,3)
subject<-c("A","A","A","B","B","C","D","D","D","E","E","E")
data<-data.frame(visit=visit,subject=subject)

I attempted to work out the latest visit ID for each subject:

tapply(visit,subject,max)

And I get this output:

A B C D E
3 2 1 3 3

I am wondering if there is any way that I can change the output such that it becomes:

A  3
B  2
C  1
D  3
E  3

Thank you

Answer 1

You can easily do this in base R with stack:

stack(tapply(visit, subject, max))
#   values ind
# 1      3   A
# 2      2   B
# 3      1   C
# 4      3   D
# 5      3   E

(Note: In this case, the values for "visit" and "subject" aren't actually coming from your data.frame. Just thought you should know!)

---

(Second note: You could also do data.frame(as.table(tapply(visit, subject, max))) but that is more deceptive than using stack so may lead to less readable code later on.)

Answer 2

It may feel dirty, but using the base function as.matrix (or matrix for that matter) will give you what you need.

> as.matrix(tapply(visit,subject,max))
  [,1]
A    3
B    2
C    1
D    3
E    3

Answer 3

And a dplyr solution would be:

library(dyplr)  
data %>% group_by(subject) %>% summarize(max = max(visit))
## Source: local data frame [5 x 2]

##   subject max
## 1       A   3
## 2       B   2
## 3       C   1
## 4       D   3
## 5       E   3

Answer 4

You can try aggregate

aggregate(visit~subject, data, max)
#   subject visit
#1       A     3
#2       B     2
#3       C     1
#4       D     3
#5       E     3

Or from tapply

res <- tapply(visit,subject,max)
data.frame(subject=names(res), visit=res)

Or data.table

library(data.table)
setDT(data)[, list(visit=max(visit)), by=subject]