subset() 删除向量上的属性；如何维护/持久化它们? [英] subset() drops attributes on vectors; how to maintain/persist them?

问题描述

假设我有一个向量，其中设置了一些属性:

Let's say I have a vector where I've set a few attributes:

vec <- sample(50:100,1000, replace=TRUE)
attr(vec, "someattr") <- "Hello World"

当我对向量进行子集化时，属性会被删除.例如:

When I subset the vector, the attributes are dropped. For example:

tmp.vec <- vec[which(vec > 80)]
attributes(tmp.vec) # Now NULL

有没有一种方法可以对属性进行子集化和持久化，而不必将它们保存到另一个临时对象?

Is there a way to, subset and persist attributes without having to save them to another temporary object?

奖励:在哪里可以找到这种行为的文档?

Bonus: Where would one find documentation of this behaviour?

推荐答案

我会为 [ 或 subset() 写一个方法(取决于你如何设置子集)并安排它保留属性.这将需要一个 "class" 属性也添加到您的向量中，以便进行调度.

I would write a method for [ or subset() (depending on how you are subsetting) and arrange for that to preserve the attributes. That would need a "class" attribute also adding to your vector so that dispatch occurs.

vec <- 1:10
attr(vec, "someattr") <- "Hello World"
class(vec) <- "foo"

此时，子集删除属性:

> vec[1:5]
[1] 1 2 3 4 5

如果我们添加一个方法 [.foo 我们可以保留属性:

If we add a method [.foo we can preserve the attributes:

`[.foo` <- function(x, i, ...) {
    attrs <- attributes(x)
    out <- unclass(x)
    out <- out[i]
    attributes(out) <- attrs
    out
}

现在保留了所需的行为

> vec[1:5]
[1] 1 2 3 4 5
attr(,"someattr")
[1] "Hello World"
attr(,"class")
[1] "foo"

<小时>

以及奖金问题的答案:

---

And the answer to the bonus question:

来自 ?"[" 在详细信息部分:

From ?"[" in the details section:

子集(空索引除外)将删除除名称、dim 和 dimnames 之外的所有属性.

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