FYI, this is FAQ 7.1
Wacek Kusnierczyk wrote:
Barry Rowlingson wrote:
2008/11/3 <[EMAIL PROTECTED]>:
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
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