How to check existence of an input argument for R functions

Question

I have a function defined as

myFun <- function(x, y, ...) {
  # using exists
  if (exists("z")) { print("exists z!") }
  # using missing
  try(if (!missing("z")) { print("z is not missing!") }, silent = TRUE)
  # using get
  try(if (get("z")) { print("get z!") }, silent = TRUE)

  # anotherFun(...)
}

In this function, I want to check whether user input "z" in the argument list. How can I do that? I tried exists("z"), missing("z"), and get("z") and none of them works.

Answer 1

@Sacha Epskamp has a pretty good solution, but it doesn't always work. The case where it fails is if the "z" argument is passed in as NULL...

# Sacha's solution
myFun <- function(x, y, ...) { 
  args <- list(...)
  exist <- !is.null(args[['z']])
  return(exist)
}

myFun(x=3, z=NULL) # FALSE, but should be TRUE!


# My variant
myFun2 <- function(x, y, ...) {
  args <- list(...)
  exist <- "z" %in% names(args)
  exist
}

myFun2(x=3, z=NULL) # TRUE

Answer 2

I think you're simply looking for hasArg

myFun <- function(x, y, ...) { 
  hasArg(z)
}

> myFun(x=3, z=NULL)
[1] TRUE

From ?hasArg:

The expression hasArg(x), for example, is similar to !missing(x), with two exceptions. First, hasArg will look for an argument named x in the call if x is not a formal argument to the calling function, but ... is. Second, hasArg never generates an error if given a name as an argument, whereas missing(x) generates an error if x is not a formal argument.

Answer 3

There might be instances when you might not want to call list(...), since this will evaluate all the expressions in the dots. For example,

myFun <- function(x, y, ...){
  myArgs <- list(...)
  zInArgs <- ("z" %in% names(myArgs))
  return(zInArgs)
}

myFun(x = 2, y = "Happy", z = list(rep(rnorm(2e6), 100)))

This will take a long time. Instead, use match.call():

myFun <- function(x, y, ...){
  myArgs <- match.call()
  zInArgs <- ("z" %in% names(myArgs))
  return(zInArgs)
}

myFun(x = 2, y = "Happy", z = list(rep(rnorm(2e6), 100)))

The first example is still chugging away on my machine, while the second example should take practically no time at all.

EDIT:

To answer the comment from @CarlWitthoft:

R> system.time(
+   (myAns <- myFun(x = 2, y = "Happy", z = list(rep(rnorm(2e6), 100))))
+ )
   user  system elapsed 
      0       0       0 
R> myAns
[1] TRUE

Answer 4

Here is a way I often do it. First convert ... to a list, then check if the elements are not NULL:

myFun <- function(x, y, ...) { 
args <- list(...)
exist <- !is.null(args[['z']])
return(exist)
}

Some results:

> myFun()
[1] FALSE
> myFun(z=1)
[1] TRUE