Check frequency of data.table value in other data.table

Question

 library(data.table)
 DT1 <- data.table(num = 1:6, group = c(\"A\", \"B\", \"B\", \"B\", \"A\", \"C\"))
 DT2 <- data.table(group = c(\"A\", \"B\", \"C\"))
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Answer 1

This is how I would do it: first count the number of times each group appears in DT1, then simply join DT2 and DT1.

require(data.table)
DT1 <- data.table(num = 1:6, group = c("A", "B", "B", "B", "A", "C"))
DT2 <- data.table(group = c("A", "B", "C"))

#solution:
DT1[,num_counts:=.N,by=group] #the number of entries in this group, just count the other column
setkey(DT1, group)
setkey(DT2, group)
DT2 = DT1[DT2,mult="last"][,list(group, popular = (num_counts >= 2))]

#> DT2
#   group popular
#1:     A    TRUE
#2:     B    TRUE
#3:     C   FALSE