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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