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, 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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Thomas Lumley Assoc. Professor, Biostatistics
tlum...@u.washington.edu University of Washington, Seattle
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