R in a Nutshell, 2nd Edition

Arguments

A function definition in R includes the names of arguments. Optionally, it may include default values. If you specify a default value for an argument, then the argument is considered optional:

> f <- function(x, y) {x + y}
> f(1,2)
[1] 3
> g <- function(x, y=10) {x + y}
> g(1)
[1] 11

If you do not specify a default value for an argument, and you do not specify a value when calling the function, you will get an error if the function attempts to use the argument:^[25]

> f(1)
Error in f(1) : 
  element 2 is empty;
   the part of the args list of '+' being evaluated was:
   (x, y)

In a function call, you may override the default value:

> g(1, 2)
[1] 3

In R, it is often convenient to specify a variable-length argument list. You might want to pass extra arguments to another function, or you may want to write a function that accepts a variable number of arguments. To do this in R, you specify an ellipsis (...) in the arguments to the function.^[26]

As an example, let’s create a function that prints the first argument and then passes all the other arguments to the summary function. To do this, we will create a function that takes one argument: x. The arguments specification also includes an ellipsis to indicate that the function takes other arguments. We can then call the summary function with the ellipsis as its argument:

> v <- c(sqrt(1:100))
> f <- function(x,...) {print(x); summary(...)}
> f("Here is the summary for v.", v, digits=2) [1] "Here is the summary for v." Min. 1st Qu. Median Mean ...