R: frequency with group by ID [duplicate]

Answer 1

library(data.table)
setDT(x)[, .(Freq = .N), by = .(ID, Cont)]

#     ID Cont Freq
# 1:  1    a    2
# 2:  1    b    1
# 3:  2    a    1
# 4:  2    c    1
# 5:  2    d    1