R语言定义对于回答这些类型的问题很方便:

http://cran.r-project.org/doc/manuals/R-lang.html#Indexing

R has three basic indexing operators, with syntax displayed by the following examples x[i] x[i, j] x[[i]] x[[i, j]] x$a x$"a" For vectors and matrices the [[ forms are rarely used, although they have some slight semantic differences from the [ form (e.g. it drops any names or dimnames attribute, and that partial matching is used for character indices). When indexing multi-dimensional structures with a single index, x[[i]] or x[i] will return the ith sequential element of x. For lists, one generally uses [[ to select any single element, whereas [ returns a list of the selected elements. The [[ form allows only a single element to be selected using integer or character indices, whereas [ allows indexing by vectors. Note though that for a list, the index can be a vector and each element of the vector is applied in turn to the list, the selected component, the selected component of that component, and so on. The result is still a single element.

双括号访问列表元素，而单括号返回一个包含单个元素的列表。

lst <- list('one','two','three')

a <- lst[1]
class(a)
## returns "list"

a <- lst[[1]]
class(a)
## returns "character"

哈德利·维克汉姆:

我(蹩脚的外观)修改显示使用tidyverse / purrr:

R语言定义对于回答这些类型的问题很方便:

http://cran.r-project.org/doc/manuals/R-lang.html#Indexing

R has three basic indexing operators, with syntax displayed by the following examples x[i] x[i, j] x[[i]] x[[i, j]] x$a x$"a" For vectors and matrices the [[ forms are rarely used, although they have some slight semantic differences from the [ form (e.g. it drops any names or dimnames attribute, and that partial matching is used for character indices). When indexing multi-dimensional structures with a single index, x[[i]] or x[i] will return the ith sequential element of x. For lists, one generally uses [[ to select any single element, whereas [ returns a list of the selected elements. The [[ form allows only a single element to be selected using integer or character indices, whereas [ allows indexing by vectors. Note though that for a list, the index can be a vector and each element of the vector is applied in turn to the list, the selected component, the selected component of that component, and so on. The result is still a single element.

只是在这里添加[[也用于递归索引。

@JijoMatthew在回答中暗示了这一点，但没有深入探讨。

正如在?"[["中所指出的，像x[[y]]这样的语法，其中长度(y) > 1被解释为:

x[[ y[1] ]][[ y[2] ]][[ y[3] ]] ... [[ y[length(y)] ]]

注意，这并没有改变关于[和[[]之间的主要区别，即前者用于子集设置，后者用于提取单个列表元素。

例如,

x <- list(list(list(1), 2), list(list(list(3), 4), 5), 6)
x
# [[1]]
# [[1]][[1]]
# [[1]][[1]][[1]]
# [1] 1
#
# [[1]][[2]]
# [1] 2
#
# [[2]]
# [[2]][[1]]
# [[2]][[1]][[1]]
# [[2]][[1]][[1]][[1]]
# [1] 3
#
# [[2]][[1]][[2]]
# [1] 4
#
# [[2]][[2]]
# [1] 5
#
# [[3]]
# [1] 6

要得到值3，我们可以这样做:

x[[c(2, 1, 1, 1)]]
# [1] 3

回到上面@JijoMatthew的答案，回想一下r:

r <- list(1:10, foo=1, far=2)

特别是，这解释了我们在误用[[时往往会得到的错误，即:

r[[1:3]]

r[[1:3]]中的错误:递归索引在级别2失败

由于这段代码实际上试图求值r[[1]][[2]][[3]]，并且r的嵌套在第一级停止，因此通过递归索引提取的尝试在[[2]]失败，即在第2级。

r[[c("foo"， "far")]]错误:下标越界

这里，R正在寻找R [["foo"]][["far"]]，而R并不存在，所以我们得到了下标越界错误。

如果这两个错误给出相同的信息，可能会更有帮助/一致一些。

作为术语，[[操作符从列表中提取元素，而[操作符获取列表的子集。