# Re: [R] problem with repeated formal arguments and ...

From: Sundar Dorai-Raj <sundar.dorai-raj_at_pdf.com>
Date: Fri 19 Aug 2005 - 05:47:00 EST

Sundar Dorai-Raj wrote:
>
> Ross Boylan wrote:
>

```>>I want to add an argument if it is not present.  Following the Green
>>Book, p. 337:
>>test <- function(x, ...){
>>  dots <- list(...)
>>  if (!hasArg(from))
>>    from <- 0
>>  else
>>    from <- dots\$from
>>  curve(x, from=from, ...)
>>}
>>
>>
>>
>>>test(sin)
>>>test(sin, from=4)
>>
>>Error in curve(x, from = from, ...) : formal argument "from" matched by
>>multiple actual arguments
>>
>>The FAQ says, in the section on differences between R and S,
>>"R disallows repeated formal arguments in function calls."
>>
>>That seems a perfectly reasonable rule,  but how do I handle this
>>situation?
>>
```

>
>
> Hi, Ross,
>
>
> test <- function(x, from = 0, ...) {
> curve(x, from = from, ...)
> }
>
> Or another way:
>
> test <- function(x, ...) {
> dots <- list(...)
> if(!hasArg(from)) dots\$from <- 0
> dots\$expr <- x
> do.call("curve", dots)
> }
>

I didn't check this example, but for curve, since `x' is an expression, we should actually do the following:

test <- function(x, ...) {

dots <- list(...)
if(!hasArg(from)) dots\$from <- 0
dots\$expr <- substitute(x)
invisible(do.call("curve", dots))
}

test(x^3 - 3 * x, to = 5)
test(x^3 - 3 * x, from = -5, to = 5)

HTH, --sundar

> I actually prefer the latter if I'm changing many arguments. I do this
> quite often when writing custom lattice plots and I want to override
> many of the defaults.
>
> HTH,
>
> --sundar
>
> ______________________________________________
> R-help@stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help