Stavros Macrakis wrote:
>
> You might think that you can check names(xxx) to see if the slot has been
> explicitly set, but it depends on *how* you have explicitly set the slot to
> NULL:
>
> > xxx$hello <- 3
> > xxx$hello <- NULL
> > names(xxx)
> character(0) # no names -- assigning to NULL kills slot
>
kills indeed:
foo = list(bar=1)
with(foo, bar)
# 1
foo$bar = NULL
with(foo, bar)
# error: object 'bar' not found
> > xxx <- list(hello=NULL)
> > names(xxx)
> [1] "hello" # 1 name -- constructing with NULL-valued
> slot
>
but:
# cleanup -- don't do it in mission critical session
rm(list=ls())
foo
# error: object 'foo' not found
foo = NULL
foo
# NULL
that is, foo$bar = NULL kills bar within foo (even though NULL is a
valid component of lists), but foo = NULL does *not* kill foo.
> Welcome to R!
>
... and its zemanticks.
vQ
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