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

[Massimo Bressan]

#ok, finally this is my final "best and more compact" solution of the problem by merging different contributions (thanks to all indeed) 

t<-data.frame(id=c(18,91,20,68,54,27,26,15,4,97),A=c(123,345,123,678,345,123,789,345,123,789)) 
l<-sapply(unique(t$A), function(x) t$id[which(t$A==x)]) 
r<-data.frame(unique_A= unique(t$A), list_id=unlist(lapply(l, paste, collapse = ", "))) 
r 


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