[R] Lists with NULL entries

[Peter Langfelder]

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