从列表中向量中的最后一个元素创建一个列表

Question

Suppose I have a list of vectors (none of them is empty; they may be of various lengths).

How can I generate a list of elements from the ends (say) of the vectors?

For example, if my list contains 3 elements, 1:10 , 2:9 and c(3,5) , then the output list should be the list containing 10 , 9 and 5 .

In the simpler case, where all vectors are of the same length (5, say), I tried to first make the list a data.frame using as.data.frame . The problem is that my list is huge (about 1 million vectors), and as.data.frame takes ages. I thought about optimization, perhaps using this answer , but anyway I think that it results a wrong data structure - instead of having 5 columns with 1 million values each, it creates a data.frame with a million columns.

So I should probably look somewhere else. Any ideas?

Answer 1

如果x是原子向量的列表，则应用：

 sapply(x, function(elem) elem[length(elem)])

Answer 2

Use tail :

> sapply(list(1:10,2:9,c(3,5)),tail,1)
[1] 10  9  5

And @Bach is correct about speed. While tail is terse, it is slow:

set.seed(1)
a <- list()
for(i in 1:1e3)
    a[[i]] <- sample(1:100, sample(1:100), TRUE)

library("microbenchmark")
microbenchmark(sapply(a,tail,1), sapply(a, function(elem) elem[length(elem)]))
# Unit: milliseconds
#                                          expr       min        lq    median        uq       max neval
#                            sapply(a, tail, 1) 22.998933 24.790584 26.103241 27.797130 79.489209   100
#  sapply(a, function(elem) elem[length(elem)])  1.546477  1.787224  1.951572  2.137856  6.106236   100