[Rd] assigning NULLs to elements of a list

[Oleg Sklyar]

Dear developers:

I have just came across an (unexpected to me) behaviour of lists when 
assigning NULLs to list elements. I understand that a NULL is a valid R 
object, thus assigning a NULL to a list element should yield exactly the 
same result as assigning any other object. So I was surprised when 
assigning a NULL in fact removed the element from the list. Is this an 
intended behaviour? If so, does anybody know where is it documented and 
what is a good way around?

Thanks for help,
Oleg Sklyar

Here goes an example, the inline C-code does exactly what I would prefer 
R were doing, but both R examples do remove the element 'b':

x = list(a=c(1L,2L), b=matrix(runif(4),2,2), c=LETTERS[1:3])

library(inline)
code="
   SEXP res;
   PROTECT(res = Rf_duplicate(x));
   SET_VECTOR_ELT(res,1,R_NilValue);
   UNPROTECT(1);
   return res;
"
setnull = cfunction(signature(x="list"),code)
setnull(x)

# $a
# [1] 1 2
# $b
# NULL
# $c
# [1] "A" "B" "C"

y = x
x[[2]] = NULL
x
# $a
# [1] 1 2
# $c
# [1] "A" "B" "C"

x = y
x$b = NULL
x
# $a
# [1] 1 2
# $c
# [1] "A" "B" "C"

 > sessionInfo()
R version 2.6.1 (2007-11-26)
i686-pc-linux-gnu

locale:
LC_CTYPE=en_GB.UTF-8;LC_NUMERIC=C;LC_TIME=en_GB.UTF-8;LC_COLLATE=en_GB.UTF-8;LC_MONETARY=en_GB.UTF-8;LC_MESSAGES=en_GB.UTF-8;LC_PAPER=en_GB.UTF-8;LC_NAME=C;LC_ADDRESS=C;LC_TELEPHONE=C;LC_MEASUREMENT=en_GB.UTF-8;LC_IDENTIFICATION=C

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

other attached packages:
[1] inline_0.3.3

loaded via a namespace (and not attached):
[1] rcompgen_0.1-17


-- 
Dr Oleg Sklyar * EBI-EMBL, Cambridge CB10 1SD, UK * +44-1223-494466