- 如果列表中间的某一元素没有输入，则将该元素自动保存为NULL
a <- NULL
a[[1]] <- matrix(1:12, ncol = 3)
a[[3]] <- c("B","C","F")
结果输出如下：
a
[[1]]
[,1] [,2] [,3]
[1,] 1 5 9
[2,] 2 6 10
[3,] 3 7 11
[4,] 4 8 12
[[2]]
NULL
[[3]]
[1] “B” “C” “F”
- 将列表中某一元素置为NULL，则将列表长度自动缩减，将那个NULL元素自动抹除
a <- NULL
a[[1]] <- matrix(1:12, ncol = 3)
a[[2]] <- 1:4
a[[3]] <- c("B","C","F")
a[[2]] <- NULL
length(a)
a
结果输出如下：
length(a)
[1] 2
a
[[1]]
[,1] [,2] [,3]
[1,] 1 5 9
[2,] 2 6 10
[3,] 3 7 11
[4,] 4 8 12
[[2]]
[1] “B” “C” “F”