On 10/25/2009 03:43 PM, mau...@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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