data.table內組ID

Question

我有一個data.table與n個分組變量（在這種情況下2）。我想爲每個組添加一個標識符列，如下面所需的輸出所示。我試圖:=：.N`和我爲什麼不工作，但不知道如何做到這一點：

library(data.table) 
dat <- data.table::data.table(
    w = 1:16, 
    x = LETTERS[1:2], 
    y = 1:4 
)[, w := NULL][order(x, y)] 


##  x y 
## 1: A 1 
## 2: A 1 
## 3: A 1 
## 4: A 1 
## 5: A 3 
## 6: A 3 
## 7: A 3 
## 8: A 3 
## 9: B 2 
## 10: B 2 
## 11: B 2 
## 12: B 2 
## 13: B 4 
## 14: B 4 
## 15: B 4 
## 16: B 4 


dat[, z := 1:.N, by = list(x, y)] 
dat 

所需的輸出

##  x y z 
## 1: A 1 1 
## 2: A 1 1 
## 3: A 1 1 
## 4: A 1 1 
## 5: A 3 2 
## 6: A 3 2 
## 7: A 3 2 
## 8: A 3 2 
## 9: B 2 3 
## 10: B 2 3 
## 11: B 2 3 
## 12: B 2 3 
## 13: B 4 4 
## 14: B 4 4 
## 15: B 4 4 
## 16: B 4 4 

Answer 1

dat[, z:=.GRP,by=list(x,y)] 
dat 
#  x y z 
# 1: A 1 1 
# 2: A 1 1 
# 3: A 1 1 
# 4: A 1 1 
# 5: A 3 2 
# 6: A 3 2 
# 7: A 3 2 
# 8: A 3 2 
# 9: B 2 3 
# 10: B 2 3 
# ...