R get object from global environment from function if object exists in global but use different default if not

Question

Surely this is possible, but I can't seem to find how to do it:

I'd like to have a default of a function input, but override the default and get() a variable from the global environment if it exists in the global environment. If it doesn't exist in the global environment, take the default of the function, with any setting in the function being top level and overriding them all.

Ideally it would work like this made-up non-working function:

###Does not work, desired example

myfunc <- function(x=30){
if(exists.in.global.env(x)){x <- get(x)}
###Top level is tough
if(x.is.defined.as.function.input=TRUE ????){x <- x.defined.as.input}
}else{ x <- 30} 
return(x)
}

So that if I do:

myfunc()
[1] 30

But if I create x I want it to override the default x=30 of the function and take the global environment value instead:

x <- 100
myfunc()
[1] 100

But if I have x defined inside the function, I'd like that to be top level, i.e. override everything else even if x is defined globally:

x <- 100
myfunc(x=300)
[1] 300

Thanks in advance!

Answer 1

You can modify your function to check if x exists in the .GlobalEnv and get it from there if it does, otherwise return the default value.

myfunc <- function(x = 30) {

  if ("x" %in% ls(envir = .GlobalEnv)) {
    get("x", envir = .GlobalEnv)
  } else {
    x
  }

}

So if "x" %in% ls(envir = .GlobalEnv) is FALSE it would return

myfunc()
[1] 30

If x is found it would return it. if x <- 100:

myfunc()
[1] 100

Edit after comment

If you want to make sure to only return x from the global environment if x is not specified as an argument to myfunc, you can use missing(). It returns TRUE if x was not passed and FALSE if it was:

myfunc <- function(x = 30) {

  if ("x" %in% ls(envir = .GlobalEnv) & missing(x)) {
    get("x", envir = .GlobalEnv)
  } else {
    x
  }

} 

So for your example:

x <- 100
myfunc(x=300)
[1] 300

Answer 2

The simplest method would be to set an appropriate default argument:

myfunc <- function(x=get("x", globalenv())
{
    x
}

> x <- 100
> f()
[1] 100
> f(30)
[1] 30
> rm(x)
> f()
Error in get("x", globalenv()) : object 'x' not found