标记数据中按组随机选择的N行。桌子

在数据上。表C3列中,我想标记每组随机选择的N行(C1)。已经提出了几个类似的问题 here ,则, here 和 here .但根据答案,我仍然无法找出如何为我的任务找到解决方案。

set.seed(1)    
dt = data.table(C1 = c("A","A","A","B","C","C","C","D","D","D"), 
                 C2 = c(2,1,3,1,2,3,4,5,4,5)) 

dt
    C1 C2
 1:  A  2
 2:  A  1
 3:  A  3
 4:  B  1
 5:  C  2
 6:  C  3
 7:  C  4
 8:  D  5
 9:  D  4
10:  D  5

以下是每组C1随机选择的两行的行索引(对于B组不起作用):

dt[, sample(.I, min(.N, 2)), by = C1]$V1
[1]  1  3  3  7  5 10  9

注意:对于B,只应选择一行,因为B组仅由一行组成。

以下是每组中随机选择的一行的解决方案,这通常不适用于B组:

dt[, C3 := .I == sample(.I, 1), by = C1]
dt
    C1 C2    C3
 1:  A  2 FALSE
 2:  A  1  TRUE
 3:  A  3 FALSE
 4:  B  1 FALSE
 5:  C  2  TRUE
 6:  C  3 FALSE
 7:  C  4 FALSE
 8:  D  5  TRUE
 9:  D  4 FALSE
10:  D  5 FALSE

实际上,我想把它展开N行。我试过(两排):

dt[, C3 := .I==sample(.I, min(.N, 2)), by = C1]

这当然行不通。

非常感谢您的帮助!