给出以下data.table：
library(data.table)
dt <- data.table(id=c(1,2,3,4), Year1=c(NA, 30, 40, NA), Year2=c(NA, 30, 20 ,70), Year3=c(60, 40, 0, 10), Year4=c(10,20,30,30))
dtm <- melt(dt, id.var = 'id')[order(id, variable)]
我想将索引分配给dtm，以便预期回报如下：
id variable value index
1: 1 Year1 NA NA
2: 1 Year2 NA NA
3: 1 Year3 60 1
4: 1 Year4 10 2
5: 2 Year1 30 1
6: 2 Year2 30 2
7: 2 Year3 40 3
8: 2 Year4 20 4
9: 3 Year1 40 1
10: 3 Year2 20 2
11: 3 Year3 0 3
12: 3 Year4 30 4
13: 4 Year1 NA NA
14: 4 Year2 70 1
15: 4 Year3 10 2
16: 4 Year4 30 3
我尝试了dtm[ ,index:=1:.N, by = id]，但它没有忽略NAs。
答案 0 :(得分：2)
我们需要指定＆＃39; i＆＃39;忽略NAs
dtm[!is.na(value), index := 1:.N , by = id]
dtm
# id variable value index
# 1: 1 Year1 NA NA
# 2: 1 Year2 NA NA
# 3: 1 Year3 60 1
# 4: 1 Year4 10 2
# 5: 2 Year1 30 1
# 6: 2 Year2 30 2
# 7: 2 Year3 40 3
# 8: 2 Year4 20 4
# 9: 3 Year1 40 1
#10: 3 Year2 20 2
#11: 3 Year3 0 3
#12: 3 Year4 30 4
#13: 4 Year1 NA NA
#14: 4 Year2 70 1
#15: 4 Year3 10 2
#16: 4 Year4 30 3
或者如果我们使用devel版本（来自@Frank的评论）
dtm[!is.na(value), z := rowid(id)]