我正在做一个项目，正在寻找一些帮助来使我的代码更有效地运行。我已经搜索了类似的问题，但似乎找不到任何像这个问题一样精细的问题。我提出的解决方案非常笨拙。我相信一定有一种更有效的方法来解决这个问题，比如dplyr，data.tables等。

**问题：**我有3列数据，'ids'，'x.group'和'times'。我需要为每个'x.group'提取每个'times'块中出现的前3个唯一的'ids'。

然而，我不想包含任何等于“0”的'ids'或'x.group'。我代码底部的输出会产生正确的值，但在我看来，这是一种相当尴尬的方式。
注意：在下面的代码示例中，我使用的是x.groups = ['A','B','0']，但在我的实际项目中，这些可以接受许多值，因此它们不会总是'A'或'B'，但'0'总是存在的（例如，我可以使用['A','K','0']或['M','W','0']等）。您可以在本文的底部找到示例数据集。

# find x.groups
xs <- unique(myDF$x.group)[unique(myDF$x.group) != "0"]

# DF without '0's as x.group entries
ps <- unique(myDF[which(myDF$x.group %in% xs) , c("ids","x.group","time")])

first3.x1.t1 <- ps[ps$x.group == xs[1] & ps$ids != "0" & ps$time == "1", ]$ids[1:3]
first3.x2.t1 <- ps[ps$x.group == xs[2] & ps$ids != "0" & ps$time == "1", ]$ids[1:3]
first3.x1.t2 <- ps[ps$x.group == xs[1] & ps$ids != "0" & ps$time == "2", ]$ids[1:3]
first3.x2.t2 <- ps[ps$x.group == xs[2] & ps$ids != "0" & ps$time == "2", ]$ids[1:3]
first3.x1.t3 <- ps[ps$x.group == xs[1] & ps$ids != "0" & ps$time == "3", ]$ids[1:3]
first3.x2.t3 <- ps[ps$x.group == xs[2] & ps$ids != "0" & ps$time == "3", ]$ids[1:3]

# First 3 unique ids from time block 1 for each x.group
> first3.x1.t1; first3.x2.t1;
[1] "2"  "17" "11"
[1] "5"  "10" "4"

# First 3 unique ids from time block 2 for each x.group
> first3.x1.t2; first3.x2.t2;
[1] "9"  "6"  "16"
[1] "8"  "13" "7" 

# First 3 unique ids from time block 3 for each x.group
> first3.x1.t3; first3.x2.t3;
[1] "11" "2"  "10"
[1] "1"  "3"  "13"

数据：

# create data frame
ids <- c("2","0","15","5","17","10","4","2","3","11","11","18","10","8","13","9","6","16","7","14",
     "16","7","11","12","14","5","1","11","3","2","10","17","3","13","10","17","2","10","16","10")
x.group <- c("A","A","0","B","A","B","B","A","B","A","A","0","B","B","B","A","A","A","B","B",
         "A","A","0","B","A","B","B","A","B","A","A","0","B","B","B","A","A","A","B","B")
time <- c(rep("1",13), rep("2",13), rep("3",14))

myDF <- as.data.frame(cbind(ids, x.group, time), stringsAsFactors = FALSE)
> myDF
   ids x.group time
1    2       A    1
2    0       A    1
3   15       0    1
4    5       B    1
5   17       A    1
6   10       B    1
7    4       B    1
8    2       A    1
9    3       B    1
10  11       A    1
11  11       A    1
12  18       0    1
13  10       B    1
14   8       B    2
15  13       B    2
16   9       A    2
17   6       A    2
18  16       A    2
19   7       B    2
20  14       B    2
21  16       A    2
22   7       A    2
23  11       0    2
24  12       B    2
25  14       A    2
26   5       B    2
27   1       B    3
28  11       A    3
29   3       B    3
30   2       A    3
31  10       A    3
32  17       0    3
33   3       B    3
34  13       B    3
35  10       B    3
36  17       A    3
37   2       A    3
38  10       A    3
39  16       B    3
40  10       B    3