thanks, barry and eric. I didn't do a good job---I did an awful job.
alas, should R not come with an is.defined() function? a variable may
never have been created, and this is different from a variable
existing but holding a NULL. this can be the case in the global
environment or in a data frame.
> is.null(never.before.seen)
Error: objected 'never.before.seen' not found
> is.defined(never.before.seen) ## I need this, because I do not
want an error:
[1] FALSE
your acs function doesn't really do what I want, either, because {
d=data.frame( x=1:4); exists(acs(d$x)) } tells me FALSE . I really
need
> d <- data.frame( x=1:5, y=1:5 )
> is.defined(d$x)
TRUE
> is.defined(d$z)
FALSE
> is.defined(never.before.seen)
FALSE
> is.defined(never.before.seen$anything) ## if a list does not
exist, anything in it does not exist either
FALSE
how would I define this function?
regards,
/iaw
On Wed, Nov 3, 2010 at 2:48 PM, Barry Rowlingson
<b.rowling...@lancaster.ac.uk> wrote:
> On Wed, Nov 3, 2010 at 6:17 PM, ivo welch <ivo.we...@gmail.com> wrote:
>> yikes. this is all my fault. it was the first thing that I ever
>> defined when I started using R.
>>
>> is.defined <- function(name) exists(as.character(substitute(name)))
>>
>> I presume there is something much better...
>
> You didn't do a good job testing your is.defined :)
>
> Let's see what happens when you feed it 'nonexisting$garbage'. What
> gets passed into 'exists'?
>
> acs=function(name){as.character(substitute(name))}
>
> > acs(nonexisting$garbage)
> [1] "$" "nonexisting" "garbage"
>
> - and then your exists test is doing effectively exists("$") which
> exists. Hence TRUE.
>
> What you are getting here is the expression parsed up as a function
> call ($) and its args. You'll see this if you do:
>
> > acs(fix(me))
> [1] "fix" "me"
>
> Perhaps you meant to deparse it:
>
> > acs=function(name){as.character(deparse(substitute(name)))}
> > acs(nonexisting$garbage)
> [1] "nonexisting$garbage"
> > exists(acs(nonexisting$garbage))
> [1] FALSE
>
> But you'd be better off testing list elements with is.null
>
> Barry
>
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