Thanks for the feedback, that clears things up. -Winston
On Mon, Jun 17, 2013 at 1:29 PM, <luke-tier...@uiowa.edu> wrote: > On Mon, 17 Jun 2013, Duncan Murdoch wrote: > >> On 17/06/2013 1:01 PM, Winston Chang wrote: >>> >>> I've been trying to add classes and attributes to symbol objects >>> (created with quote()), and the behavior is very strange, as >>> illustrated in the examples below. >>> >>> If symbols aren't meant to have classes and attributes attached to >>> them, then perhaps R should throw errors when you attempt to do it? >> >> >> I think this is a consequence of another strange property of symbol >> objects, namely that they are not copied on assignment. This is also true >> of environments, NULL, builtin function references, and some rarely >> encountered types like external pointers and weak references. You're >> allowed to assign attributes to all of these other than NULL, but you will >> find strange things happen if you do it (as demonstrated in your code >> below). > > > You are only because we haven't yet gotten around to preventing it -- > we will at some point. For now, just don't try to assign attributes to > symbols. > > Best, > > luke > > >> >> I thought I remembered reading a list of these in the documentation >> somewhere, but when I went to look for it just now, I couldn't find it. >> (Maybe I'm remembering a comment in the source.) It would be a useful >> addition. >> >> Duncan Murdoch >> >>> >>> >>> # Using str() strips class from object >>> x <- quote(foo) >>> class(x) <- "bar" >>> str(x) >>> # Class 'bar' symbol foo >>> str(x) >>> # symbol foo >>> >>> >>> # Attempting to overwrite doesn't affect class. >>> # str() still strips class from the object. >>> x <- quote(foo) >>> class(x) <- "bar" >>> x <- quote(foo) >>> str(x) >>> # Class 'bar' symbol foo >>> str(x) >>> # symbol foo >>> >>> >>> # Changing class of one object affects other >>> x <- quote(foo) >>> y <- quote(foo) >>> class(x) <- "bar" >>> class(y) >>> # [1] "bar" >>> str(y) >>> # Class 'bar' symbol foo >>> str(y) >>> # symbol foo >>> str(x) >>> # symbol foo >>> >>> >>> # Changing attribute of one object affects other >>> # Unlike with class, str() doesn't cause other attributes to disappear >>> x <- quote(foo) >>> y <- quote(foo) >>> attr(x, "a") <- "bar" >>> str(y) >>> # length 1 foo >>> # - attr(*, "a")= chr "bar" >>> str(y) >>> # length 1 foo >>> # - attr(*, "a")= chr "bar" >>> str(quote(foo)) >>> # length 1 foo >>> # - attr(*, "a")= chr "bar" >>> >>> >>> -Winston >>> >>> ______________________________________________ >>> R-devel@r-project.org mailing list >>> https://stat.ethz.ch/mailman/listinfo/r-devel >> >> >> ______________________________________________ >> R-devel@r-project.org mailing list >> https://stat.ethz.ch/mailman/listinfo/r-devel >> > > -- > Luke Tierney > Chair, Statistics and Actuarial Science > Ralph E. Wareham Professor of Mathematical Sciences > University of Iowa Phone: 319-335-3386 > Department of Statistics and Fax: 319-335-3017 > Actuarial Science > 241 Schaeffer Hall email: luke-tier...@uiowa.edu > Iowa City, IA 52242 WWW: http://www.stat.uiowa.edu ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
