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

[(Ted Harding)]

On 25-Oct-09 09:52:42, Patrick Burns wrote:
> '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")

Which essentially says that 

  If you want the component [x1[comp] of the list x1] to stay there
  but to be NULL, then do:

    xl[comp] <- list(NULL)

I agree that this can be very puzzling! The essential point is that
(moving to Maura's example)

  myList[2]  or, equivalently,  myList["second"]

is a LIST (whose only component is that component of the original
myList). On the other hand,

  myList[[2]]  or, equivalently,  myList$second

is NOT a list, but is the value of that component of myList:

  myList[1]
  # $first
  # [1] "aaa"

  myList[[1]]
  # [1] "aaa"

  myList["first"]
  # $first
  # [1] "aaa"

  myList[["first"]]
  # [1] "aaa"

Note the statement (under "Recursive (list-like) objects")
in ?"$" or, equivalently, ?Extract

  When either '[[' or '$' is used for replacement, a value
  of 'NULL' deletes the corresponding item of the list.

Therefore changing the value of a comnponent of a list to NULL
deletes it. So you have to work at the list level, replacing
one list by another list. Hence Patrick's tip.

Ted.

> 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]]
>> 
>> ______________________________________________
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>> 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.
>>
> 
> ______________________________________________
> 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.

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E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 25-Oct-09                                       Time: 09:30:45
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