Usage of `...` (three-dots or dot-dot-dot) in functions
REllipsisR Problem Overview
Where can I find documentation on the usage of ... in functions? Examples would be useful.
R Solutions
Solution 1 - R
The word used to describe ... is "ellipsis." Knowing this should make searching for information about the construct easier. For example, the first hit on Google is another question on this site: https://stackoverflow.com/questions/3057341/how-to-use-rs-ellipsis-feature-when-writing-your-own-function
Solution 2 - R
A little example to get you started.
f <- function(x, ...)
{
dots <- list(...) #1
if(length(dots) == 0) return(NULL)
cat("The arguments in ... are\n")
print(dots)
f(...) #2
}
f(1,2,3,"a", list("monkey"))
The function, f, stores all but the first input argument in the ellipsis variable. For accessing its contents, it is easiest to convert it to a list (1). The main use however is for passing arguments to subfunctions, which requires no conversion (2).
Solution 3 - R
You should head over to "R Language Definition", section 2.1.9 Dot-dot-dot. It comes bundled with R installation. Run help.start() in an interactive session to bring HTML help up, and click on The R Language Definition link. You can use PDF or HTML version from the main site as well.
Anyway, ... is used to match unspecified formal arguments of a function.
args(sapply)
function (X, FUN, ..., simplify = TRUE, USE.NAMES = TRUE)
NULL
sapply(mtcars, mean, trim = .5)
mpg cyl disp hp drat wt qsec vs am gear
19.200 6.000 196.300 123.000 3.695 3.325 17.710 0.000 0.000 4.000
carb
2.000
As you can see, I passed trim = .5 though it's not specified as a formal argument of sapply function.
(note that this example is trivial, you can use sapply(mtcars, median) to achieve the same result)