[Rd] The default behaviour of a missing entry in an environment
Duncan Murdoch
murdoch at stats.uwo.ca
Fri Nov 13 20:27:57 CET 2009
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] 1
>> e$b
> NULL
>>
>
> I 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
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