我有一个数据表。
set.seed(1)
DT <- data.table(tag = rep(LETTERS[1:4],each = 2, times = 3),
year = rep(1:3, each = 8), month = rep(c(1:2), 12),
value = runif(24, 1, 10))
DT
tag year month value
1: A 1 1 3.389578
2: A 1 2 4.349115
3: B 1 1 6.155680
4: B 1 2 9.173870
5: C 1 1 2.815137
6: C 1 2 9.085507
7: D 1 1 9.502077
8: D 1 2 6.947180
9: A 2 1 6.662026
10: A 2 2 1.556076
11: B 2 1 2.853771
12: B 2 2 2.589011
13: C 2 1 7.183206
14: C 2 2 4.456933
15: D 2 1 7.928573
16: D 2 2 5.479293
17: A 3 1 7.458567
18: A 3 2 9.927155
19: B 3 1 4.420317
20: B 3 2 7.997007
21: C 3 1 9.412347
22: C 3 2 2.909283
23: D 3 1 6.865064
24: D 3 2 2.129996
按year、month和-value对DT进行排序：
setorder(DT, year, month, -value)
将产生：
tag year month value
1: D 1 1 9.502077
2: B 1 1 6.155680
3: A 1 1 3.389578
4: C 1 1 2.815137
5: B 1 2 9.173870
6: C 1 2 9.085507
7: D 1 2 6.947180
8: A 1 2 4.349115
9: D 2 1 7.928573
10: C 2 1 7.183206
11: A 2 1 6.662026
12: B 2 1 2.853771
13: D 2 2 5.479293
14: C 2 2 4.456933
15: B 2 2 2.589011
16: A 2 2 1.556076
17: C 3 1 9.412347
18: A 3 1 7.458567
19: D 3 1 6.865064
20: B 3 1 4.420317
21: A 3 2 9.927155
22: B 3 2 7.997007
23: C 3 2 2.909283
24: D 3 2 2.129996
我希望结果如下：
tag year month value
1: D 1 1 9.502077
2: B 1 1 6.155680
3: B 1 2 9.173870
4: D 1 2 6.947180
5: D 2 1 7.928573
6: C 2 1 7.183206
7: D 2 2 5.479293
8: C 2 2 4.456933
9: C 3 1 9.412347
10: A 3 1 7.458567
11: A 3 2 9.927155
12: C 3 2 2.909283
结果DT将具有以下属性：在每年内仅保留在month 1中具有较大价值的两个tags。例如year 1，值较大的两个tags是D和B，所以整个year 1保持D和B。 year 2保持D和C。在每个month 1，我需要重新选择带有年份的行。
我们按"year"分组，获取"month"1的"tag"中的前两个元素(因为它已经排序(，使用%in%创建一个逻辑索引并子集行。
DT[, .SD[tag %in% head(tag[month ==1],2)], .(year)]
# year tag month value
# 1: 1 D 1 9.502077
# 2: 1 B 1 6.155680
# 3: 1 B 2 9.173870
# 4: 1 D 2 6.947180
# 5: 2 D 1 7.928573
# 6: 2 C 1 7.183206
# 7: 2 D 2 5.479293
# 8: 2 C 2 4.456933
# 9: 3 C 1 9.412347
#10: 3 A 1 7.458567
#11: 3 A 2 9.927155
#12: 3 C 2 2.909283