如何获取列表中每个元素的最后一个子元素? [英] How to get last subelement of every element of a list?

问题描述

我试图获取列表中每个子元素的 last 元素的值.列表中的元素是长度不同的向量，因此列表中元素之间的最后一个子元素的索引不是恒定的.

I am trying to get the value of the last element of every subelement of a list. The elements of the list are vectors of different length, so the index of the last sub-element is not constant between elements of the list.

我已经搜索过SO，但是只找到了那些想要获取信息的人，例如，每个元素的第二个子元素.这对我没有帮助，因为列表中的元素长度不固定.

I have searched SO but only found information for those looking to obtain, for example, the second sub-element of every element. This is not helpful for me since the elements of the list are of non-constant length.

我只对不涉及使用 for 循环的解决方案感兴趣，因为我的列表很长 >一百万.

I am only interested in solutions that do not involve using a for loop since my list is of length > 1 million.

示例数据:

x <- list(a = 1:5, b = 1:10, d = 9:11)

预期的输出是一个列表: list(5，10，11)

Expected output is a list: list(5, 10, 11)

推荐答案

我们可以使用 lapply 和 tail :

x <- list(a = 1:5, b = 1:10, d = 9:11)
last_elems <- lapply(x, tail, n = 1L)

正如评论中指出的那样，

vapply 是获取数字矢量而不是一个元素列表的好选择:

As pointed out in the comments, to obtain a numeric vector instead of a one-element list, vapply is a good choice:

last_elems <- vapply(x, tail, n = 1L, FUN.VALUE = numeric(1))

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