On Tue, May 19, 2009 at 12:07 PM, routík <zrou...@gmail.com> wrote:
> > SmoothData <- list(exists=TRUE, span=0.001)
> > exists("SmoothData$span")
> FALSE
>
As others have said, this just checks for the existence of a variable with
the (strange) name "SmoothData$span".
In some sense, in R semantics, xxx$yyy *always* exists if xxx is a list (or
other recursive object):
> xxx <- list()
> xxx$hello
NULL
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
> xxx <- list(hello=NULL)
> names(xxx)
[1] "hello" # 1 name -- constructing with NULL-valued
slot
Welcome to R!
-s
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