function to produce list of elements by group (R)

Question

I have a transient amnesia. I recall there's an R function that allows to get list of elements in a group after aggregating. To be clearer, from here:

DT<-data.table(x=rep(c("cool","uncool"),each=3),group=c(1,1,2,2,3,3))

That gives:

    x    group
   cool     1
   cool     1
   cool     2
 uncool     2
 uncool     3
 uncool     3

I would like to end up with:

 x      group
cool    1,1,2
uncool  2,3,3

I used that function in the past, but can't remember now. Anyone can help? Thank you

Maurits Evers · Accepted Answer · 2018-06-06T13:30:29.413

2

Option 1

Using aggregate

aggregate(group ~ x, DT, toString)
#       x   group
#1   cool 1, 1, 2
#2 uncool 2, 3, 3

or

aggregate(group ~ x, DT, paste0, collapse = ",")
#       x group
#1   cool 1,1,2
#2 uncool 2,3,3

Option 2

The data.table way

DT[, .(group = toString(group)), by = x]
#    x       group
#1:   cool 1, 1, 2
#2: uncool 2, 3, 3

or

DT[, .(group = paste0(group, collapse = ",")), by = x]
#        x group
#1:   cool 1,1,2
#2: uncool 2,3,3

edited Jun 06 '18 at 13:30

answered Jun 06 '18 at 13:15

Maurits Evers

45,165
4
35
59

It works, thanks! But it wasn't the one I was using. I think that function included square brackets, something like "[". – La Machine Infernale Jun 06 '18 at 13:22
@LaMachineInfernale I've added a `data.table` solution, please take a look; perhaps this was what you were after? – Maurits Evers Jun 06 '18 at 13:31

score 1 · Answer 2 · answered Jun 06 '18 at 13:42

1

Here is an option using tidyverse

library(tidyverse)
DT %>%
  group_by(x) %>%
  summarise(group = toString(group))

answered Jun 06 '18 at 13:42

akrun

789,025
32
460
575

function to produce list of elements by group (R)

2 Answers2

Option 1

Option 2