get rows of unique values by group

Question

I have a data.table and want to pick those lines of the data.table where some values of a variable x are unique relative to another variable y

It\'s possible to get

Answer 1

data.table is a bit different in how to use duplicated. Here's the approach I've seen around here somewhere before:

dt <- data.table(y=rep(letters[1:2],each=3),x=c(1,2,2,3,2,1),z=1:6) 
setkey(dt, "y", "x")
key(dt)
# [1] "y" "x"
!duplicated(dt)
# [1]  TRUE  TRUE FALSE  TRUE  TRUE  TRUE
dt[!duplicated(dt)]
#    y x z
# 1: a 1 1
# 2: a 2 2
# 3: b 1 6
# 4: b 2 5
# 5: b 3 4