[R] aggregate and list elements of variables in data.frame

[Massimo Bressan]

#given the following reproducible and simplified example 

t<-data.frame(id=1:10,A=c(123,345,123,678,345,123,789,345,123,789)) 
t 

#I need to get the following result 

r<-data.frame(unique_A=c(123, 345, 678, 789),list_id=c('1,3,6,9','2,5,8','4','7,10')) 
r 

# i.e. aggregate over the variable "A" and list all elements of the variable "id" satisfying the criteria of having the same corrisponding value of "A" 
#any help for that? 

#so far I've just managed to "aggregate" and "count", like: 

library(sqldf) 
sqldf('select count(*) as count_id, A as unique_A from t group by A') 

library(dplyr) 
t%>%group_by(unique_A=A) %>% summarise(count_id = n()) 

# thank you 


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