问题
Suppose we have an R function whose arguments must be selected out of a finite set of elements. Like qplot(..., geom=""). And geom can take only some values, like bar or point.
How can I find out all the valid values the argument of a given function may take? Apart from docs or Internet, which often miss all possible values. Perhaps, some R function can help?
回答1:
If the function of interest is defined like
f <- function(a = c("foo","bar")) {
match.arg(a)
}
i.e. when the options are defined as a vector to be later checked with match.arg function, then you could use the formals function which would give you a list of arguments with the values like in the following example
> formals(f)
$a
c("foo", "bar")
Otherwise I don't think it is possible to get all valid argument values without RTFSing.
来源：https://stackoverflow.com/questions/20352842/all-valid-values-of-an-argument-of-a-function-in-r