[R] NULL elements in lists ... a nightmare

[tlumley at u.washington.edu]

It is perhaps also worth mentioning that this is the very first question in the actual R questions section of the R FAQ.

7.1 How can I set components of a list to NULL?

You can use

      x[i] <- list(NULL)

to set component i of the list x to NULL, similarly for named components. Do not set x[i] or x[[i]] to NULL, because this will remove the corresponding component from the list.

      -thomas


On Sun, 25 Oct 2009, Jim Lemon wrote:

> On 10/25/2009 03:43 PM, mauede at alice.it wrote:
>> I can define a list containing NULL elements:
>> 
>>    
>>> myList<- list("aaa",NULL,TRUE)
>>> names(myList)<- c("first","second","third")
>>> myList
>>>      
>> $first
>> [1] "aaa"
>> $second
>> NULL
>> $third
>> [1] TRUE
>>    
>>> length(myList)
>>>      
>> [1] 3
>> 
>> However, if I assign NULL to any of the list element then such
>> element is deleted from the list:
>> 
>>    
>>> myList$second<- NULL
>>> myList
>>>      
>> $first
>> [1] "aaa"
>> $third
>> [1] TRUE
>>    
>>> length(myList)
>>>      
>> [1] 2
>>    
>>> #
>>> myList$first<- NULL
>>> myList
>>>      
>> $third
>> [1] TRUE
>>    
>>> length(myList)
>>>      
>> [1] 1
>> 
>> Instead vectors cannot include NULL element:
>> 
>>    
>>> vec<- c(TRUE,NULL,FALSE)
>>> vec
>>>      
>> [1]  TRUE FALSE
>>    
>>> length(vec)
>>>      
>> [1] 2
>>    
>>> vec[1]<- NULL
>>>      
>> Error in vec[1]<- NULL : replacement has length zero
>> 
>> Is the above shown behaviour of list data structures to be expected ?
>> I took me a lot of sweat to figure out this wierd behaviour was the cause 
>> of a bug
>> in my big program.
>> In general, if I have a list with some elements initialized to NULL, that 
>> can be changed
>> dynamically, then how can I reinitialize such elements to NULL without 
>> deleting them
>> from the list ?
>> 
>>    
> Hi Maura,
> As Patrick indicated, you can assign NULL to an existing element of a list 
> with:
>
> mylist[2]<-list(NULL)
>
> but only with the single bracket extractor. If you try this:
>
> mylist$second<-list(NULL)
> #OR
> mylist[[2]]<-list(NULL)
>
> you will get the unexpected result of the element becoming a list with a 
> component that is NULL. This also happens if you try to add a new element:
>
> mylist[4]<-list(NULL)
>
> is okay, but:
>
> mylist[[4]]<-list(NULL)
> #OR
> mylist$fourth<-list(NULL)
>
> lands you in the same pickle. The single bracket extractor gets you the list 
> component, but the double brackets or the equivalent extraction by name gets 
> you what is _in_ that component. Instead of "make this list component contain 
> NULL" the command is saying "make this list component contain a list that 
> contains NULL". When you just assign NULL, it is like saying "make this 
> component of the list NULL" (i.e. not there).
>
> A vector is atomic, all components must be of the same data type. So any 
> _something_ (e.g. numeric, character, logical) is not the same as _nothing_ 
> (NULL). The concatenation function, when confronted with two somethings 
> separated by a nothing, simply drops the nothing.
>
> Jim
>
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Thomas Lumley			Assoc. Professor, Biostatistics
tlumley at u.washington.edu	University of Washington, Seattle