assigning NULLs to elements of a list

Hmm, I think the pretty traditional R style does the job...

a = list(1,2,3)
a[-2]

So I really do not see a good reason for doing a[2] = NULL instead of a
= a[-2]


Jeffrey J. Hallman wrote:

From your tone, I gather you don't much like this behavior, and I can see your

point, as it not very intuitive that setting a list element to NULL deletes
any existing element at that index.  But is there a better way to delete an
element from a list?  Maybe there should be.

Jeff

Prof Brian Ripley <ripley at stats.ox.ac.uk> writes:

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?

Yes, it was apparently intended: R has long done this.

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

is what I think you are intending.

See e.g. the comment in subassign.c

         /* If "val" is NULL, this is an element deletion */
         /* if there is a match to "nlist" otherwise "x" */
         /* is unchanged.  The attributes need adjustment. */

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