问题
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examplefn <- function(x = NULL, ...){str(x)}
I'm trying to get this function to honor the implicit x = NULL. Consider the following:
For a call using both x and ... this results as expected in:
> examplefn(1,2)
num 1
If using an explicit x = NULL, the behavior also is as expected:
> examplefn(x = NULL,2)
NULL
However, when attempting (and expecting the usage of x = NULL from the function definition, I get:
> examplefn(2)
num 2
Implying, that the call is evaluated by argument order, disregarding the x = NULL definition.
How can the latter be prevented?
回答1:
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.
If you want to circumvent this, here are a few approaches:
1. By creating two functions
fun0 <- function (x, ...) str(x)
fun1 <- function (...) fun0(NULL, ...)
fun1(2)
# NULL
2. Another approach is by naming you arguments, e.g.
fun2 <- function (x = NULL, y) str(x)
fun2(y = 2)
# NULL
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. Finally, here is also a (trivial) possibility - setting x <- NULL inside the function
fun4 <- function (...) {
x <- NULL
str(x)
}
fun4(2)
# NULL
But I am assuming you have reasons to want x to be an argument.
来源：https://stackoverflow.com/questions/54436901/r-ellipse-and-null-in-function-parameters