如何在R中的data.table的两个列表列上运行set操作？

Question

我正在尝试在两列上应用相交或者说一列和一个列表。它适用于我是否有一个列的多个组但不是如果我只有一个，我不能理解为什么？它会抛出警告，不知何故它会以不同的方式列出...

使用：

$ awk 'NR==2{next}                         # skip second line
       r {t=$1; $1=$2; $2=t}               # if r (reverse) flag is set, swap fields
       1;                                  # print line
       /^#Start hosts lists/{r=1}' file    # if the line matches set the r flag


#hosts entry
#Start hosts lists

Server1 10.0.0.1
Server2 10.0.0.2
Server1 10.0.0.3
Server2 10.0.0.4

但不适用于：

dt <- data.table(a = c("a","a", "b","b", "c"), b = c("PKR","AFB", "ICH", 
"PKR", "GHI"))
dt <- dt[, c :=.(list(unique(b))), by=a] 
y <- nrow(dt)
dt <- dt[, i := mapply(intersect, c, rep(list(c("AFB","PKR")), y))]


dt <- data.table(a = c("a","a", "b","b"), b = c("PKR","AFB", "ICH", "PKR"))
dt <- dt[, c :=.(list(unique(b))), by=a] 
y <- nrow(dt)
dt <- dt[, i := mapply(intersect, c, rep(list(c("AFB","PKR")), y))]

dt <- data.table(a = c("a","a", "b"), b = c("PKR","AFB", "ICH"))
dt <- dt[, c :=.(list(unique(b))), by=a] 
y <- nrow(dt)
dt <- dt[, i := mapply(intersect, c, rep(list(c("AFB","PKR")), y))]

有人可以分享这是如何运作的吗？或者我错过了什么。