[R] How to detect NULL list element?

[Wacek Kusnierczyk]

Barry Rowlingson wrote:
> 2008/11/3  <rkevinburton at charter.net>:
>
>   
>> So how do I detect the NULL at r[1]?
>>     
>
>  How can you detect what is not there?
>
>  Single square brackets on a list give you a list. Double square
> brackets give you the elements.
>
>  is.null(r[[1]]) should be TRUE.
>   

interestingly:

(l = list(1, NULL))
[[1]]
[1] 1

[[2]]
NULL


l = list(1); l[[2]] = NULL; l
[[1]]
[1] 1


l = as.list(1:3); l[[2]] = NULL; l
[[1]]
[1] 1

[[2]]
[1] 2


you can have NULL as an element on a list, but assigning NULL to an
element of a list removes the element rather than makes it a NULL. 

i find it more coherent if l[i] would remove the element (as it does)
while l[[i]] would assign NULL to it (as it doesn't), OR if list(1,
NULL) would return a list of 1 element.  note, x = NULL *assigns* NULL
to x rather than removes x:

x
Error: object "x" not found

x = NULL; x
NULL

vQ