[Rd] Re: [R] More user-friendly error message needed.
Douglas Bates
bates at stat.wisc.edu
Wed Apr 7 23:13:05 CEST 2004
"Warnes, Gregory R" <gregory_r_warnes at groton.pfizer.com> writes:
> Perhaps one could create a utility function
>
> has.element <- function(list, name) name %in% names(list)
>
> and then have $ generate a warning (not an error!) when the named element
> does not exist.
Because of partial matching, list$name can return a result when
your has.element function returns FALSE.
> lst = list(foo = 1:3, bar = LETTERS[1:3])
> lst$f
[1] 1 2 3
> has.element <- function(list, name) name %in% names(list)
> has.element(lst, "f")
[1] FALSE
Before we go too far in speculating about what the $ operator should
or should not return when the name is not matched, let's all remember
that the semantics of the $ operator explicitly state that it should
return NULL and, as Duncan wrote, there are many, many places in the
base code for R that depend upon this behavior. Trust me - you really
don't want to change this.
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