[R] Testing for existence inside a function

[Gabor Grothendieck]

Try this modification:

> chk <- function(x) exists(deparse(substitute(x)), parent.env(environment()))
> ab <- 1
> chk(ab)
[1] TRUE
> exists("x")
[1] FALSE
> chk(x)
[1] FALSE



On 5/15/07, Talbot Katz <topkatz at msn.com> wrote:
> Hi.
>
> Thanks once more for the swift response.  This solution works pretty well.
> The only small glitch is if I pass the function an argument with the same
> name as the function argument.  That is, suppose "x" is the argument name in
> my user-defined function, and that object "x" does not exist.  If I call the
> function f(x), i.e., using the non-existent object x as the argument value,
> then the function says that x exists.
>
> Here is my example log:
>
> >chkex5 <- function(objn){
> + c(exob=exists(deparse(substitute(objn))))
> + }
> >exists("objn")
> [1] FALSE
> >chkex5(objn)
> exob
> TRUE
> >
>
> But I suppose I can live with this.  Thanks again!
>
>
> --  TMK  --
> 212-460-5430    home
> 917-656-5351    cell
>
>
>
> >From: "Liaw, Andy" <andy_liaw at merck.com>
> >To: "Talbot Katz" <topkatz at msn.com>,r-help at stat.math.ethz.ch
> >Subject: RE: [R] Testing for existence inside a function
> >Date: Tue, 15 May 2007 11:41:17 -0400
> >
> >Just need a bit more work:
> >
> >R> f <- function(x) exists(deparse(substitute(x)))
> >R> f(y)
> >[1] FALSE
> >R> y <- 1
> >R> f(y)
> >[1] TRUE
> >R> f(z)
> >[1] FALSE
> >
> >Andy
> >
> >From: Talbot Katz
> > >
> > > Hi, Andy.
> > >
> > > Thank you for the quick response!  Unfortunately, none of
> > > these are exactly
> > > what I'm looking for.  I'm looking for the following:
> > > Suppose object y
> > > exists and object z does not exist.  If I pass y as the value of the
> > > argument to my function, I want to be able to verify, inside
> > > my function,
> > > the existence of y; similarly, if I pass z as the value of
> > > the argument, I
> > > want to be able to see, inside the function, that z doesn't exist.
> > >
> > > The missing function just checks whether the argument is
> > > missing; in my
> > > case, the argument is not missing, but the object may not
> > > exist.  And the
> > > way you use the exists function inside the user-defined
> > > function doesn't
> > > test the argument to the user-defined function, it's just
> > > hard-coded for the
> > > object y.  So I'm sorry if I wasn't clear before, and I hope
> > > this is clear
> > > now.  Perhaps what I'm attempting to do is unavailable
> > > because it's a bad
> > > programming paradigm.  But even an explanation if that's the
> > > case would be
> > > appreciated.
> > >
> > > --  TMK  --
> > > 212-460-5430        home
> > > 917-656-5351        cell
> > >
> > >
> > >
> > > >From: "Liaw, Andy" <andy_liaw at merck.com>
> > > >To: "Talbot Katz" <topkatz at msn.com>,r-help at stat.math.ethz.ch
> > > >Subject: RE: [R] Testing for existence inside a function  [Broadcast]
> > > >Date: Tue, 15 May 2007 11:03:12 -0400
> > > >
> > > >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
> > > > >
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