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

[Patrick Burns]

'The R Inferno' page 59.


Patrick Burns
patrick at burns-stat.com
+44 (0)20 8525 0696
http://www.burns-stat.com
(home of "The R Inferno" and "A Guide for the Unwilling S User")

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 ?
> 
> Thank you in advance,
> Maura
> 
> 
> 
> 
> 
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