Question

I have a data frame like this:

user event score
1      2     0.5
1      3     0.2
2      3     0.7
3      1     0.9
3      4     0.1

and I want to get the result:

user event
1    2,3
2    3
3    1,4

How can I do this in R?
Sometimes, I think R’s lacking of base data structures in programming languages such as Java or Python makes my work inconvinient.

Answer 1

Assuming your data.frame is called mydata, do:

aggregate(event ~ user, mydata, I)

to get a list that retains the numeric representation or

aggregate(event ~ user, mydata, paste, collapse = ", ")

Which pastes the values to create a character vector.

Here’s a demonstration of how these are stored differently:

x1 <- aggregate(event ~ user, mydata, paste, collapse =",")
x2 <- aggregate(event ~ user, mydata, I)
x1$event
# [1] "2,3" "3"   "1,4"
x2$event
# $`0`
# [1] 2 3
# 
# $`1`
# [1] 3
# 
# $`2`
# [1] 1 4
# 

I personally prefer using I to paste so that I can use lapply() on that column if I need to later.