On 11/13/2009 2:03 PM, Trishank Karthik Kuppusamy wrote:
Greetings everyone,I have a question about the default behaviour of a missing entry in an environment. Let us look at the following sequence of R statements:e <- new.env() e$a <- 1 e$a[1] 1e$bNULLI think I understand the logic for returning NULL to a missing entry in an environment, but I do not think that it is fully justified. I am sure that the R developers must have seen this argument before, but I wish to call for attention to this problem again, because I think that it is important to the default safety of the R programming language.
You get the same behaviour when asking for a nonexistent element of a list, or a nonexistent attribute. If you want stricter checking, don't use $, use get():
> get("b", e)
Error in get("b", e) : object 'b' not found
or check first with exists():
> exists("b", e)
[1] FALSE
I suppose that one could argue that a good R programmer must be careful not to use NULL in any of his environment entries, but I think it is better to remove altogether this burden from the programmer and simply raise a good, old-fashioned exception when the "$" operator encounters a missing entry in an environment.
But then it would be inconsistent with what it does in other situations. Duncan Murdoch
The biggest advantage is that it will easily eliminate a whole class of programming error. The biggest disadvantage is that it is not backwards-compatible with old R programs. I suppose a personal solution would be to simply redefine the "$" operator in my programs. However, I really do think that the default safety of an R environment matters very much. At the very least, it would be nice to be able to configure the safety of a new environment, perhaps through a parameter. -Trishank ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
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