Get the argument names of an R function

Question

For an arbitrary function

f <- function(x, y = 3){
  z <- x + y
  z^2
}

I want to be able take the argument names of f

> argument_names(f)
[1] "x" "y"

Is this possible?

Answer 1

formalArgs and formals are two functions that would be useful in this case. If you just want the parameter names then formalArgs will be more useful as it just gives the names and ignores any defaults. formals gives a list as the output and provides the parameter name as the name of the element in the list and the default as the value of the element.

f <- function(x, y = 3){
  z <- x + y
  z^2
}

> formalArgs(f)
[1] "x" "y"
> formals(f)
$x


$y
[1] 3

My first inclination was to just suggest formals and if you just wanted the names of the parameters you could use names like names(formals(f)). The formalArgs function just is a wrapper that does that for you so either way works.

Edit: Note that technically primitive functions don't have "formals" so this method will return NULL if used on primitives. A way around that is to first wrap the function in args before passing to formalArgs. This works regardless of it the function is primitive or not.

> # formalArgs will work for non-primitives but not primitives
> formalArgs(f)
[1] "x" "y"
> formalArgs(sum)
NULL
> # But wrapping the function in args first will work in either case
> formalArgs(args(f))
[1] "x" "y"
> formalArgs(args(sum))
[1] "..."   "na.rm"