r - ..1 和 ..2 在 R 中代表什么？

Question

这个问题在这里已经有了答案:





Two dots, ".." in R

(3 个回答)


6年前关闭。




引用 R 语言定义:

Notice that identifiers starting with a period are not by default listed by the ls function and that ‘...’ and ‘..1’, ‘..2’, etc. are special.

The following identifiers have a special meaning and cannot be used for object names if else repeat while function for in next break TRUE FALSE NULL Inf NaN NA NA_integer_ NA_real_ NA_complex_ NA_character_ ... ..1 ..2 etc.

然而，它没有提供任何进一步的细节。谁能详细说明一下？

Answer 1

这些用于从 ... 位置提取值函数的参数。请参阅下面的示例:

myfun <- function(...) {
   list(a = ..1, b = ..2, c = ..3)
}

myfun(1,2,3)
# $a
# [1] 1
# $b
# [1] 2
# $c
# [1] 3

myfun(3,2,1)
# $a
# [1] 3
# $b
# [1] 2
# $c
# [1] 1

myfun(1:5, "hello", letters[1:3])
# $a
# [1] 1 2 3 4 5
# $b
# [1] "hello"
# $c
# [1] "a" "b" "c"

如果您尝试从控制台调用其中之一，则此用途变得显而易见:

> ..1
Error: ..1 used in an incorrect context, no ... to look in