[R] Lists with NULL entries

[Joshua Wiley]

Sorry, that was a really half-hearted reply.  This will create a new
list that is the old list shifted down (and should be much faster than
the for loop too).

lst <- list(NULL,2)
lst2 <- vector("list", length(lst) + 1)
lst2[2:length(lst2)] <- lst
lst
lst2

If you really need to use a for loop, maybe try filling it with NAs
instead of NULLs?

Josh

On Mon, Sep 20, 2010 at 7:26 PM, Joshua Wiley <jwiley.psych at gmail.com> wrote:
> Hello Peter,
>
> This is because assigning a value of NULL removes that element of the
> list.  I am not quite sure what the reference for that is.  I remember
> reading it in the documentation once though.  I looked through ?list
> to no avail.  At any rate, to avoid it, you would have to assign
> something besides NULL.
>
> HTH,
>
> Josh
>
> On Mon, Sep 20, 2010 at 7:06 PM, Peter Langfelder
> <peter.langfelder at gmail.com> wrote:
>> Hello,
>>
>> I encountered a weird problem. Consider the following code that takes
>> a list "lst" and shifts all elements one index up (for example, to
>> make space for a new first element):
>>
>> lst = list(1,2)
>> ll = length(lst);
>> for (i in ll:1)
>>   lst[[i+1]] = lst[[i]];
>> lst
>>
>> If you run it, you get the expected result
>>
>> [[1]]
>> [1] 1
>>
>> [[2]]
>> [1] 1
>>
>> [[3]]
>> [1] 2
>>
>> Now I change the input such that the first element is a NULL.
>>
>> lst = list(NULL,2)
>> ll = length(lst);
>> for (i in ll:1)
>>   lst[[i+1]] = lst[[i]];
>> lst
>>
>> When you run the code, you get
>>
>> [[1]]
>> NULL
>>
>> [[2]]
>> [1] 2
>>
>> i.e. the shift did not happen. Why is that and how can the shift be
>> made to work correctly in the presence of NULL elements in the list?
>>
>> Thanks,
>>
>> Peter
>>
>> ______________________________________________
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>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>
>
>
> --
> Joshua Wiley
> Ph.D. Student, Health Psychology
> University of California, Los Angeles
> http://www.joshuawiley.com/