problem with repeated formal arguments and ...

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,

Add "from" to your function:

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

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problem with repeated formal arguments and ...

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