Count number of records and generate row number within each group in a data.table
Using .N...
DT[ , `:=`( COUNT = .N , IDX = 1:.N ) , by = VAL ]
# VAL COUNT IDX
# 1: 1 3 1
# 2: 2 4 1
# 3: 2 4 2
# 4: 3 3 1
# 5: 1 3 2
# 6: 3 3 2
# 7: 3 3 3
# 8: 2 4 3
# 9: 2 4 4
#10: 1 3 3
.N is the number of records in each group, with groups defined by "VAL".