[R] How to detect NULL list element?

[Wacek Kusnierczyk]

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
>>
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Wacek Kusnierczyk, MD PhD

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