Re: [R] Variable passed to function not used in function in select=... in subset

From: Duncan Murdoch <murdoch_at_stats.uwo.ca>
Date: Tue, 11 Nov 2008 09:37:57 -0500

On 11/11/2008 8:53 AM, hadley wickham wrote:
> On Mon, Nov 10, 2008 at 1:04 PM, Wacek Kusnierczyk
> <Waclaw.Marcin.Kusnierczyk_at_idi.ntnu.no> wrote:

>> pardon me, but does this address in any way the legitimate complaint of
>> the rightfully confused user?
>>
>> consider the following:
>>
>> d = data.frame(a=1, b=2)
>> a = c("a", "b")
>> z = a
>> # that is, both a and z are c("a", "b")
>>
>> subset(d, select=z)
>> # gives two columns, since z is a two element vector whose elements are
>> valid column names
>>
>> subset(d, select=a)
>> # gives one column, since 'a' (but not a) is a valid column name
>>
>> subset(d, select=c(a,b))
>> # gives two columns
>>
>>
>> this is certainly what the authors intended, and they may have good
>> grounds for this smart design.  but this must break the expectation of a
>> naive (r-naive, for that matter) user, who may otherwise have excellent
>> experience in using a functional programming language, e.g., scheme.
>> (especially scheme, where symbols and expressions are first-class
>> objects, yet the distinction between a symbol or an expression and their
>> referent is made painfully clear, perhaps except for when one hacks with
>> macros.)
>>
>> the examples above illustrate the notorious problem with r that one can
>> never tell whether 'a' means "the value referred to with the identifier
>> 'a'" or "the symbol 'a'", unless one gets ugly surprises and is forced
>> to study the documentation.  and even then one may not get a clear answer.

>
> I agree, with some caveats. There are basically two uses of R: as a
> interactive data analysis package and as a statistical programming
> language. These uses come into conflict: in the interactive
> environment, you want to minimise typing so that you can be as speedy
> as possible. It doesn't matter if R occasionally makes a wrong guess
> when you have specified something implicitly, because you can fix it
> on the fly. When you are programming, you care less about saving
> typing and more about reproducibility. You want to be explicit so
> your function is robust to widely varying inputs, even if it means you
> have to type a lot more. You see this tension in quite a few places:
>
> * drop = T
> * functions that return different types of output (e.g. sapply)
> depending on input parameters
> * partial matching of argument names
> * using unevaluated expressions instead of strings (e.g. library, subset, ...)
>
> These are all things that are helpful for interactive use, but make
> life as a programmer more difficult. I find the last one particularly
> frustrating because it means it is very difficult to program with some
> functions (i.e subset) without resorting to complex quoting,
> substituting and evaluating tricks. I have tried to steer away from
> this technique in my packages, and where it's just too convenient for
> interactive use, insulating the deparsing into special functions that
> the data analyst must use (e.g. aes() in ggplot, and .() in plyr),
> along with providing alternatives for the programmer.
>
> I don't understand why you're getting so much push-back on this issue.
> R is a fantastic language, but it has some genuinely nasty corners.
> In my opinion, this is one of them.

I think your analysis is correct, that the goals of casual use and programming are inconsistent. But in general I think there's always going to be support for providing alternative ways that are programmer-safe.

For instance, library( foo, character.only=TRUE) says that foo is a character vector, not the name of a package. I don't know of anything that subset() provides that is not available in other ways (I think of it as purely a convenience function, and my first piece of advice to Karl was not to use it). However, if there really is something there, then it would be worthwhile pointing that out, and either modifying subset() to make it safe, or providing an alternative function.

I think this tension is a fundamental part of the character of S and R.   But it is also fundamental to R that there are QC tests that apply to code in packages: so writing new tests that detect dangerous usage (e.g. to disallow partial name matching) would be another way to improve reliability. Writing a test for misuse of drop=TRUE seems quite hard, but there are probably ways a debugger could do it: e.g. to tag the invocation as to whether any indices were dropped on the first call, and then warn if the result isn't the same on every subsequent call).

Conceivably Karl's problem could be detected in the same way: tag each name in the expression as to whether it was found in the data frame or some other environment, and then warn if that tag ever changes. Or maybe the test should just warn that subset() is a convenience function, not meant for programming.

Duncan Murdoch



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