Re: [R] How to detect NULL list element?

From: Wacek Kusnierczyk <Waclaw.Marcin.Kusnierczyk_at_idi.ntnu.no>
Date: Mon, 03 Nov 2008 15:27:53 +0100

Erik Iverson wrote:
> FYI, this is FAQ 7.1

... which says what you should and should not do, but without any a justification for the design. coherently, x[i] = list(NULL) should do the same as x[[i]] = NULL, if only for symmetry with x[i] = list(1) and x[[1]] = 1, for example.

vQ

>
> Wacek Kusnierczyk wrote:
>> 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
>>
>> ______________________________________________
>> R-help_at_r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.

-- 
-------------------------------------------------------------------------------
Wacek Kusnierczyk, MD PhD

Email: waku_at_idi.ntnu.no
Phone: +47 73591875, +47 72574609

Department of Computer and Information Science (IDI)
Faculty of Information Technology, Mathematics and Electrical Engineering (IME)
Norwegian University of Science and Technology (NTNU)
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Room itv303

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Received on Mon 03 Nov 2008 - 14:29:50 GMT

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