[R] Lists with NULL entries

[Joshua Wiley]

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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>



-- 
Joshua Wiley
Ph.D. Student, Health Psychology
University of California, Los Angeles
http://www.joshuawiley.com/