如何在矢量列表上应用索引向量？ [英] How do I apply an index vector over a list of vectors?

问题描述

我想将一个长索引向量（50+非连续整数）应用于一长列向量（包含100多个名称的50多个字符向量），以便检索特定值（作为列表，向量或数据）帧）。

I want to apply a long index vector (50+ non-sequential integers) to a long list of vectors (50+ character vectors containing 100+ names) in order to retrieve specific values (as a list, vector, or data frame).

简化示例如下：

> my.list <- list(c("a","b","c"),c("d","e","f"))
> my.index <- 2:3

期望输出

[[1]]
[1] "b"
[[2]]
[1] "f"
##or
[1] "b"
[1] "f"
##or
[1] "b" "f"

我知道我可以使用以下每个元素获得相同的值：

I know I can get the same value from each element using:

> lapply(my.list, function(x) x[2])
##or
> lapply(my.list,'[', 2)

我可以从第二个和第三个值中提取每个元素：

I can pull the second and third values from each element by:

> lapply(my.list,'[', my.index)
[[1]]
[1] "b" "c"

[[2]]
[1] "e" "f"

##or
> for(j in my.index) for(i in seq_along(my.list)) print(my.list[[i]][[j]])
[1] "b"
[1] "e"
[1] "c"
[1] "f"

我不知道如何从每个元素中提取一个值。

I don't know how to pull just the one value from each element.

我一直在寻找几天但没有发现任何这样做的例子，但似乎相当直接。我错过了一些明显的东西吗？

I've been looking for a few days and haven't found any examples of this being done, but it seems fairly straight forward. Am I missing something obvious here?

谢谢你，
Scott

Thank you, Scott

推荐答案

每当遇到类似 lapply 的问题但涉及多个并行列表/向量时，请考虑 Map 或 mapply （ Map 只是一个包装 mapply 的包装 SIMPLIFY = FALSE 硬编码。

Whenever you have a problem that is like lapply but involves multiple parallel lists/vectors, consider Map or mapply (Map simply being a wrapper around mapply with SIMPLIFY=FALSE hardcoded).

试试这个：

Map("[",my.list,my.index)
#[[1]]
#[1] "b"
#
#[[2]]
#[1] "f"

..或：

mapply("[",my.list,my.index)
#[1] "b" "f"

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