R列表获取每个元素的第一项 [英] R list get first item of each element

问题描述

对于某人来说，这应该很容易回答，但是我在任何地方都找不到答案.

This should probably be very easy for someone to answer but I have had no success on finding the answer anywhere.

我试图从R中的列表返回列表中每个元素的第一项.

I am trying to return, from a list in R, the first item of each element of the list.

> a
[1] 1 2 3
> b
[1] 11 22 33
> c
[1] 111 222 333
> d <- list(a = a,b = b,c = c)
> d
$a
[1] 1 2 3

$b
[1] 11 22 33

$c
[1] 111 222 333

基于上面我的列表d的构造，我想返回一个带有三个值的向量:

Based on the construction of my list d above, I want to return a vector with three values:

return  1 11 111

推荐答案

sapply(d, "[[", 1)应该可以解决问题.

一些解释:

sapply:遍历列表中的元素
[[:是子集函数.因此，我们要求Sapply在每个列表元素上使用子集函数.
1:是传递给"[["

sapply: iterates over the elements in the list
[[: is the subset function. So we are asking sapply to use the subset function on each list element.
1 : is an argument passed to "[["

事实证明，可以以传统方式调用"["或"[["，这可能有助于说明这一点:

It turns out that "[" or "[[" can be called in a traditional manner which may help to illustrate the point:

x <- 10:1
"["(x, 3)
 # [1] 8

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