R: using ellipsis argument (...)

ellipsis, r

Solution

AN ACTUAL ANSWER:

You can do this through a bit of trickery. First, define your function as before, but include a list with your default arguments inside the function. Then you can parse whatever arguments come in through `...` as a list, replace the defaults with anything in `...` and then pass the updated list of arguments through `do.call`.

myplot <- function(x, ...) {
    args1 <- list(cex=4, main="Default Title") # specify defaults here
    inargs <- list(...)
    args1[names(inargs)] <- inargs
    do.call(plot, c(list(x=x), args1))
}

myplot(x=1:3) # call with default arguments
myplot(x=1:3, cex=2, main="Replacement", xlab="Test xlab") # call with optional arguments

EARLIER COMMENTARY:

The problem here can be seen through a few example functions:

myplot1 <- function(x, ... ) {
    plot(x, cex= 1.5, ... )
}

myplot2 <- function(x, cex=3, ... ) {
    plot(x, cex=cex, ... )
}

myplot3 <- function(x, ... ) {
    plot(x, ... )
}

myplot1(1:3, cex=3) # spits your error
myplot2(1:3, cex=3) # works fine
myplot3(1:3, cex=3) # works fine

In `myplot2`, you specify a default value of `cex` but can change it. In `myplot3`, `cex` is simply passed through. If you run `myplot2` with two `cex` arguments, you'll see what's happening with your function (`myplot1`):

myplot2(1:3, cex=3, cex=1.5) # same error as above

So, you're probably best to avoid setting any defaults in `plot()`, so then you can pass anything through the `...` in `myplot`.

Problem

I want to create a wrapper function replacing some of the default arguments. Here the core of the problem I'm struggling with: ``` Error in localWindow(xlim, ylim, log, asp, ...) : formal argument "cex" matched by multiple actual arguments ``` Now a bit of a context. Suppose I define a wrapper function for plot like this: ``` myplot <- function(x, ... ) { plot(x, cex= 1.5, ... ) } ``` If I call `myplot( 1:10, cex= 2 )` I will get the above error. I know I can turn `...` to a list ``` l <- list(...) ``` and then I could do ``` if( is.null( l[["cex"]] ) ) l[["cex"]] <- 2 ``` However, how can I "insert" this list back to the ellipsis argument? Something like (I know this won't work): ``` ... <- l ``` EDIT: I could use defaults in `myplot` definition (as suggested in the answer from @Thomas), but I don't want to: the function interface will become cluttered. I guess I could define a helper function like that: ``` .myfunchelper <- function( x, cex= 2.0, ... ) { plot( x, cex= cex, ... ) } myfunc <- function( x, ... ) { .myfunchelper( x, ... ) } ``` But (i) it is less elegant and (ii) doesn't satisfy my curiosity.

Original source

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