How to combine values in column B based on values in Column A using R?

Question

I'm trying to combine values in Item.ID column below based on ID value in ID column, the values in the Item.ID column can be seperated using a comma:

Dataframe (AS):

AS <- data.frame("Index" = c(1,1,2,2,2,3,4), "Item.ID" = c("A1","C2","A3","U4","M5","K6","Y9"))

Required Output

AS_Wide <- data.frame("Index" = c(1,2,3,4), "Item.ID" = c("A1,C2","A3,U4,M5","K6","Y9"))

Answer 1

Using base R, with just aggregate and paste :

AS.Wide <- aggregate(AS$Item.ID, by=list(Index=AS$Index), paste, collapse=",")

If you want to keep the "Item.ID" variable name, you will need to change it:

names(AS.Wide)[2] <- "Item.ID"

Answer 2

Using Tidyverse/dplyr:

Group by Index,

Arrange them by Item.ID as well so the result is alphabetized but that is up to you.

paste all the Item.ID's together by collapsing them with a ",".

library(dplyr)
AS_Wide <- AS %>% 
  group_by(Index) %>% 
  arrange(Item.ID) %>% 
  summarize(Item.ID = paste(Item.ID, collapse = ","))