“[.data.frame”的正文中的“.1”

When I look at the contents of [[.data.frame on my PC, this is what I get:

当我看到[[.data.frame在我的电脑上的内容时，这就是我得到的：

> get("[[.data.frame")
function (x, ..., exact = TRUE) 
{
    na <- nargs() - (!missing(exact))
    if (!all(names(sys.call()) %in% c("", "exact"))) 
        warning("named arguments other than 'exact' are discouraged")
    if (na < 3L) 
        (function(x, i, exact) if (is.matrix(i)) 
            as.matrix(x)[[i]]
        else .subset2(x, i, exact = exact))(x, ..., exact = exact)
    else {
        col <- .subset2(x, ..2, exact = exact)
        i <- if (is.character(..1)) 
            pmatch(..1, row.names(x), duplicates.ok = TRUE)
        else ..1
        .subset2(col, i, exact = exact)
    }
}
<environment: namespace:base>

I've gotten used to ..., but this is the first time I saw ..1 and ..2. A quick search both in R help and Google returned mostly rubbish, since the dots are often interpreted as placeholders, so I'm hoping someone here can give me a pointer? Or am I missing something dreadfully obvious? Where do these come from and how can I use them?

我已经习惯了...，但这是我第一次看到..1和..2。快速搜索R帮助和谷歌返回主要是垃圾，因为点通常被解释为占位符，所以我希望有人在这里可以给我一个指针？还是我错过了一些非常明显的东西？这些来自何处以及如何使用它们？

2 个解决方案

#1

This is a way to reference the 1st, 2nd, ... elements of the special pairlist that is .... So ..1 is the way to refer to the first element of ..., ..2 refers to the second element of ... and so on.

这是引用特殊pairlist的第1，第2 ......元素的一种方式....所以..1是引用......的第一个元素的方式。，。2指的是第二个元素...的元素等等。

This is mentioned in the R Internals manual in section 1.5.2 Dot-dot-dot arguments, the relevant bit of which is:

这在R.2 Internals手册的1.5.2点 - 点点参数中提到，其相关位是：

The value of ... is a (special) pairlist whose elements are referred to by the special symbols ..1, ..2, ... which have the DDVAL bit set: when one of these is encountered it is looked up (via ddfindVar) in the value of the ... symbol in the evaluation frame.

......的值是一个（特殊的）pairlist，其元素由特殊符号引用..1，..2，...具有DDVAL位设置：当遇到其中一个时，它被查找（通过ddfindVar）在评估框架中的...符号的值。

#2

To add to Gavin's answer:

添加到Gavin的答案：

They are also mentioned briefly in the help page for reserved words (?Reserved).

它们也在保留字的帮助页面中简要提及（？保留）。

A really simple example of usage is

一个非常简单的使用示例是

f <- function(...) print(..1)
f(x = 99)  #prints 99
f()        #throws an error

#1