R ellipsis - further arguments passed to or from other methods

Question

I understand what dot-dot-dot means in general. I understand how to use it when I want to create my own function with unknown number of parameters.

I do not understand how it works for example at function variable.names(). When I execute ?variable.names, there is written the following:

... further arguments passed to or from other methods.

What does it really mean? I do not know what I can pass there. How and where will be these passed arguments used.

Answer 1

The ellipsis argument allows one to pass arguments to a downstream function. We'll illustrate with a simple R function as follows.

testfunc <- function(aFunction,x,...) {
     aFunction(x,...)
}
aVector <- c(1,3,5,NA,7,9,11,32)

# returns NA because aVector contains NA values
testfunc(mean,aVector)

# use ellipsis in testfunc to pass na.rm=TRUE to mean()
testfunc(mean,aVector,na.rm=TRUE)

...and the output:

> testfunc <- function(aFunction,x,...) {
+      aFunction(x,...)
+ }
> aVector <- c(1,3,5,NA,7,9,11,32)
> 
> # returns NA because aVector contains NA values
> testfunc(mean,aVector)
[1] NA
> # use ellipsis in testfunc to pass na.rm=TRUE to mean()
> testfunc(mean,aVector,na.rm=TRUE)
[1] 9.714286