[R] Testing for existence inside a function
Liaw, Andy
andy_liaw at merck.com
Tue May 15 18:36:08 CEST 2007
Another thing to watch out for is that an argument to a function can be
an expression (or even literal constants), instead of just the name of
an object. exists() wouldn't really do the right thing. I'm not sure
how to properly do the exhaustive check.
Andy
From: 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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