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

[Jim Lemon]

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