为什么在purrr中的lambda函数中使用省略号...时必须将其包装在c()中 [英] why ellipsis ... have to be wrapped inside c() when used in lambda functions in purrr

问题描述

此问题可能称为此问题的第3部分.

我知道在 tidyverse 中使用的lambda函数中的参数，尤其是 purrr 函数中的参数写为-

I know that arguments in lambda functions when used in tidyverse especially purrr functions are written as -

• .只有1个参数时
• .x & .y 当有2
时
• 或 .. 1 ， .. 2 ，以此类推.2 个参数

• . when there is only 1 argument
• .x & .y when there are 2
• OR ..1, ..2 so on when there are > 2 arguments

在链接的问题中，我了解到，如果必须同时传递所有参数，则可以使用省略号，即 ... .

In the linked question, I learnt that if all the arguments have to be simultaneously passed we may use ellipsis i.e. ....

但是我的问题是，为什么这样的 ... 仅当包裹在 c()中时才起作用，而当这样使用时却不起作用.在以下两种语法中，第二种有效，而第一种无效?

But my question, why such ... work only when wrapped inside a c() and doesn't work when used as such. In the below two syntaxes, second one works while first one doesn't?

#1 this doesn't work

pmap_df(iris[1:4], ~...)

Error in .f(Sepal.Length = .l[[1L]][[i]], Sepal.Width = .l[[2L]][[i]],  : 
  '...' used in an incorrect context



#2 this however, works and returns the first argument after converting it to a tibble
pmap_df(iris[1:4], ~ c(...))

#see
identical(pmap_df(iris[1:4], ~c(...)), iris[1:4] %>% as_tibble())
[1] TRUE

有人可以解释吗?

推荐答案

The offical documentation has this to be said about the ellipsis:

The components of ‘...’ can be accessed in the usual pairlist manner from C code, but is not easily accessed as an object in interpreted code.

这可能是您问题的答案的一部分:省略号是一个不易访问的对象.

So that's possibly part of the answer to your question: The ellipsis is a not easily accessed object.

您可能必须进行野生动物园之旅，深入了解事物的C代码，以了解有关此内容的更多信息.

You would likely have to go on a safari trip down into the C-code of things to learn more about that.

现在，作为不易访问的对象， ... 当然不符合列表或向量的要求，而这正是pmap_df想要的.尽管从技术上讲，它可能可以.

Now being a not easily accessed object the ... certainly doesn't qualify as a list or vector, which is what pmap_df wants. Although technically it probably could.

尽管看到如何使用〜创建单行功能主体，但真正的原因可能是它本身并未首先考虑省略号.

However seeing how ~ is used to create one-liner function bodies, the real reason is probably that it itself does not consider the ellipsis in the first place.

考虑:

f <- function( ... ) {
    ~...
}

f(a=1,b=2) ## returns just `~...` , no trace of a and b in the formula's environment either.

现在考虑:〜........ .效果一样好.

Now consider: ~........ . Works just as fine.

〜愉快地使用任意数量的点或其他字母数字字符，〜....... 或〜aaa ......例如bb ... cc ... ，有效地表明它并不关心查看省略号的正常方法，而只是将其视为名称的一部分.

~ happily takes any number of dots or other alfa num characters, ~....... or ~aaa......bb...cc... for example, effectively demonstrating it doesn't care about the otherwise normal way to look at the ellipsis and just treats them as part of a name.

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