R中使用双方括号的子集列表
在阅读了有关R中的子集列表的多个线程之后，我试图通过在Rstudio中四处闲逛来完全理解这一概念.我以为我理解了这个概念，直到遇到以下问题:
After reading several threads about subsetting lists in R, I tried to fully grasp this notion by fooling around in Rstudio. I thought I understood the concept until I came across the following:
x <- list(list(list(1), 2), list(list(list(3), 4), 5), 6)
为什么x[[1]]返回一个包含两个元素的列表，而x[[1]][[1]]也返回一个列表呢?
Why is it that x[[1]] returns a list with two elements and x[[1]][[1]] returns a list too?
使用两个额外的空格查看代码，以使结构更清晰:
Look at the code with two extra spaces to help make the structure more clear:
x <- list( list( list(1), 2), list(list(list(3), 4), 5), 6)
x的第一个元素是:list( list(1), 2)#显然有两个元素的列表
The first element of x is: list( list(1), 2) # clearly a list with two elements
该列表的第一个元素是:list(1)#也是一个列表，但只有一个元素
And the first element of that list is: list(1) # also a list but with one element