[R] Dealing with an argument missing in ...

[Gavin Simpson]

On Wed, 2010-03-10 at 11:28 +0100, Christophe Genolini wrote:
> Hi the list
> 
> I define a S3 function that can have a various number of argument using 
> '...'
> When some arguments are missing, I would like to give them some default 
> value:
>  
> func.numeric <- function(x,...){
>    if(missing(y)){y<-3}
> }
>  
> But it does not works...

If you don't want to specify the extra arguments for some reason in this
method, then you can process the '...'

func.numeric <- function(x, ...) {
    dots <- list(...)
    if(!("y" %in% names(dots))) {
        y <- 3
    } else {
        y <- dots[["y"]]
    }
    return(x + y)
}

> func.numeric(x = 3)
[1] 6
> func.numeric(x = 3, y = 10)
[1] 13

> I precise that I can not put 'y' explicitly in the argument list since 
> func is an S3 function, I cannot change its arguments.

You can, as long as you include all the arguments of the generic:

foo <- function(x, ...) {
    UseMethod("foo")
}

foo.numeric <- function(x, y, ...) {
    return(x + y)
}

> foo(x = 3, y = 10)
[1] 13

HTH

G

> Any suggestions?
>  
> Thanks
> Christophe
> 
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