函数中“…”（三点或点对点）的用法

Question

在哪里可以找到有关...in函数用法的文档？例子将是有用的。

Answer 1

一个让您入门的小例子。f <- function(x, ...){&nbsp; dots <- list(...)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;#1&nbsp; if(length(dots) == 0) return(NULL)&nbsp;&nbsp; cat("The arguments in ... are\n")&nbsp; print(dots)&nbsp; f(...)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; #2}f(1,2,3,"a", list("monkey"))函数，f将第一个输入参数以外的所有参数存储在省略号变量中。为了访问其内容，最简单的方法是将其转换为列表（1）。但是，主要用途是将参数传递给子函数，而无需进行转换（2）。

Answer 2

您应该转到“ R语言定义”的第2.1.9节Dot-dot-dot。它与R安装捆绑在一起。help.start()在交互式会话中运行以启动HTML帮助，然后单击“ R语言定义”链接。您也可以从主站点使用PDF或HTML版本。无论如何，...用于匹配函数的未指定形式参数。args(sapply)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;function (X, FUN, ..., simplify = TRUE, USE.NAMES = TRUE)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp;NULL&nbsp; &nbsp;sapply(mtcars, mean, trim = .5)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp;&nbsp; &nbsp; mpg&nbsp; &nbsp; &nbsp;cyl&nbsp; &nbsp; disp&nbsp; &nbsp; &nbsp; hp&nbsp; &nbsp; drat&nbsp; &nbsp; &nbsp; wt&nbsp; &nbsp; qsec&nbsp; &nbsp; &nbsp; vs&nbsp; &nbsp; &nbsp; am&nbsp; &nbsp; gear&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp;&nbsp;19.200&nbsp; &nbsp;6.000 196.300 123.000&nbsp; &nbsp;3.695&nbsp; &nbsp;3.325&nbsp; 17.710&nbsp; &nbsp;0.000&nbsp; &nbsp;0.000&nbsp; &nbsp;4.000&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp;&nbsp; &nbsp;carb&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp;&nbsp; 2.000&nbsp;如您所见，trim = .5尽管没有将其指定为sapply函数的形式参数，但我还是通过了。（请注意，此示例是微不足道的，可以sapply(mtcars, median)用来实现相同的结果）