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
On Tue, 12 Feb 2008, Oleg Sklyar wrote:> 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?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. */> > 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 > > ______________________________________________ > R-devel at r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel >-- Brian D. Ripley, ripley at stats.ox.ac.uk Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595
On Tue, Feb 12, 2008 at 11:06:59AM +0000, Oleg Sklyar wrote:> 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]] = NULLHi Oleg, To do this I use x[2] <- list(NULL) In particular I do this when constructing lists to act as the 'what' argument for scan(), where a NULL element means that the column should be skipped, whereas obviously a non-existent element won't serve that purpose. I would be interested to hear whether this is the 'proper' way to construct such lists. Dan> 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 > > ______________________________________________ > R-devel at r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel
Oleg Sklyar wrote:> 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?This topic is discussed in the R FAQ, in section 7.1 "How can I set components of a list to NULL?" It also shows up in section 3.3.3, in the context of incompatibilities with S. - Steve
On Tue, 12 Feb 2008, Oleg Sklyar wrote:> 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?Yes.> If so, does anybody know where is it documented and > what is a good way around? >One place is FAQ 7.1 -thomas Thomas Lumley Assoc. Professor, Biostatistics tlumley at u.washington.edu University of Washington, Seattle