On Nov 3, 2010, at 3:32 PM, ivo welch wrote:
thanks, barry and eric. I didn't do a good job---I did an awful job.
is.defined(never.before.seen$anything) ## if a list does not
exist, anything in it does not exist either
Except the $ function return NULL rather than an error and you already
said you were willing to accept a NULL value as being different than
not-existing.
You may want to look at the difference between `$` and `[` methods of
accessing values.
You can test for never.before.seen as an object
is.defined <- function(x) !("try-error" %in% class(try(x)) )
But it won't give your desired result on d$never.before.seen which
does not throw an error. For that you would need an additional test of
the sort Iverson is suggesting.
--
David.
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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