R help - May 2007 - Testing for existence inside a function

Hi.

I'm having trouble testing for existence of an object inside a function.

Suppose I have a function:

f<-function(x){
...
}

and I call it with argument y:

f(y)

I'd like to check inside the function whether argument y exists.  Is this 
possible, or do I have to either check outside the function or pass the name 
of the argument as a separate argument?

If I do exists(x)  or exists(eval(x)) inside the function and y does not 
exist, it generates an error message.  If I do exists("x") it says
that x
exists even if y does not.  If I had a separate argument to hold the text 
string "y" then I could check that.  But is it possible to check the 
existence of the argument inside the function without passing its name as a 
separate argument?

Thanks!

--  TMK  --
212-460-5430	home
917-656-5351	cell

Not sure which one you want, but the following should cover it:

R> f <- function(x) c(x=missing(x), y=exists("y"))
R> f(1)
    x     y 
FALSE FALSE 
R> f()
    x     y 
 TRUE FALSE 
R> y <- 1
R> f()
   x    y 
TRUE TRUE 
R> f(1)
    x     y 
FALSE  TRUE 

Andy 

From: Talbot Katz
> 
> Hi.
> 
> I'm having trouble testing for existence of an object inside 
> a function.
> 
> Suppose I have a function:
> 
> f<-function(x){
> ...
> }
> 
> and I call it with argument y:
> 
> f(y)
> 
> I'd like to check inside the function whether argument y 
> exists.  Is this 
> possible, or do I have to either check outside the function 
> or pass the name 
> of the argument as a separate argument?
> 
> If I do exists(x)  or exists(eval(x)) inside the function and 
> y does not 
> exist, it generates an error message.  If I do exists("x") it 
> says that x 
> exists even if y does not.  If I had a separate argument to 
> hold the text 
> string "y" then I could check that.  But is it possible to check
the
> existence of the argument inside the function without passing 
> its name as a 
> separate argument?
> 
> Thanks!
> 
> --  TMK  --
> 212-460-5430	home
> 917-656-5351	cell
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide 
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
> 
> 
> 

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Talbot Katz wrote:
> 
> I'm having trouble testing for existence of an object inside a
function.
> 
No, you are having trouble testing for existence of an object
_before_ the function is called :-)
> Suppose I have a function:
> 
> f<-function(x){
> ...
> }
> 
> and I call it with argument y:
> 
> f(y)
> 
> I'd like to check inside the function whether argument y exists.
>
This can't be done, because the error happens before f is called.

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.

Alberto Monteiro