假设我有一个x如下列表：
library(purrr)
set.seed(4152)
x <- rerun(5, a = sample(c("A","B","C","D"),1), b = c(1,2,3,4,5))
x
[[1]]
[[1]]$a
[1] "B"
[[1]]$b
[1] 1 2 3 4 5
[[2]]
[[2]]$a
[1] "B"
[[2]]$b
[1] 1 2 3 4 5
[[3]]
[[3]]$a
[1] "C"
[[3]]$b
[1] 1 2 3 4 5
[[4]]
[[4]]$a
[1] "C"
[[4]]$b
[1] 1 2 3 4 5
[[5]]
[[5]]$a
[1] "A"
[[5]]$b
[1] 1 2 3 4 5
我想要keep列表x中的所有元素$a %in% c("A","C")，所以我尝试了：
x_sub …