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] <- NULLError 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 [[alternative HTML version deleted]]
'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 > > > > > > [[alternative HTML version deleted]] > > ______________________________________________ > R-help at r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. >
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
tlumley at u.washington.edu
2009-Oct-26 01:21 UTC
[R] NULL elements in lists ... a nightmare
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, 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
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
Thomas Lumley Assoc. Professor, Biostatistics
tlumley at u.washington.edu University of Washington, Seattle