R-函数参数中的椭圆(`...`)和`NULL` [英] R - Ellipse (`...`) and `NULL` in function parameters

问题描述

构想

examplefn <- function(x = NULL, ...){str(x)}

我正在尝试使用此函数来兑现隐式的 x = NULL .请考虑以下内容:

I'm trying to get this function to honor the implicit x = NULL. Consider the following:

对于同时使用 x 和 ... 的呼叫，此结果在预期的情况下出现:

For a call using both x and ... this results as expected in:

> examplefn(1,2)
num 1

如果使用显式的 x = NULL ，则该行为也符合预期:

If using an explicit x = NULL, the behavior also is as expected:

> examplefn(x = NULL,2)
NULL

但是，当尝试(并期望从函数定义中使用 x = NULL 时，我得到:

However, when attempting (and expecting the usage of x = NULL from the function definition, I get:

> examplefn(2)
num 2

简单地说，该调用是按参数顺序求值的，而忽略了 x = NULL 的定义.

Implying, that the call is evaluated by argument order, disregarding the x = NULL definition.

如何预防后者?

推荐答案

仅当未提供 x 值时，才使用定义 x = NULL .因此，在编写 examplefn(2)时， R 读取的是 examplefn(x = 2)(因为 x 是参数数字1)和结果.

The definition x = NULL is only used if no x value is provided. So when writing examplefn(2) what R reads is examplefn(x = 2) (as x is argument number 1) and thus the result.

如果您想避免这种情况，可以采取以下几种方法:

If you want to circumvent this, here are a few approaches:

1..通过创建两个函数

fun0 <- function (x, ...) str(x) 
fun1 <- function (...) fun0(NULL, ...)
fun1(2)
# NULL

2..另一种方法是为您命名参数，例如

2. Another approach is by naming you arguments, e.g.

fun2 <- function (x = NULL, y) str(x)
fun2(y = 2) 
# NULL

3..也许对您来说最方便的另一种方法是简单地对参数重新排序，请参见

3. Another way, maybe the most convenient for you, is simply to reorder the arguments, see

fun3 <- function (..., x = NULL) str(x)
fun3(2)  
# NULL

4.最后，这也是一种(简单的)可能性-在函数内部设置 x<-NULL

4. Finally, here is also a (trivial) possibility - setting x <- NULL inside the function

fun4 <- function (...) {
  x <- NULL
  str(x)
}
fun4(2)    
# NULL

但是我假设您有理由希望 x 作为参数.

But I am assuming you have reasons to want x to be an argument.

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