[R] Testing for existence inside a function

[Alberto Monteiro]

Duncan Murdoch wrote:
> 
>> Try this:
>> 
>> f <- function(x) x + 1
>> f(y.does.not.exist)
>> y.does.not.exist
>> 
>> The error message is (almost) the same, and it happens when
>> parsing the line. There's no way to change f to change this.
> 
> That description is true in some languages, but not in R.  R doesn't 
> check that args to functions are valid until it needs to use them. 
>  For example:
> 
>  > f <- function(y) 1  # doesn't care if y exists
>  > f(y.does.not.exist)
> [1] 1
>
Ok, I guess R optimizes every call to f, ignoring its arguments
unless needed.

f <- function(y) 1 # doesn't care if y exists
g <- function() cat("g was called\n")
f(g())
[1] 1
# g was not called

Another test:

f1 <- function(x, y) if (x == 0) y else 1
f1(1, y.does.not.exist)
f1(1, g())

The y-argument is never called.

So maybe it _might_ be possible to test if y exists inside the 
function...

Alberto Monteiro