[R] variable scope

[R. Michael Weylandt]

On Tue, Aug 28, 2012 at 5:13 PM, R. Michael Weylandt
<michael.weylandt at gmail.com> wrote:
> On Tue, Aug 28, 2012 at 4:55 PM, Sam Steingold <sds at gnu.org> wrote:
>>> * R. Michael Weylandt <zvpunry.jrlynaqg at tznvy.pbz> [2012-08-28 13:45:35 -0500]:
>>>
>>>> always you shouldn't need manual garbage collection.
>>
>> my observation is that gc in R sucks.
>> (it cannot release small objects).
>> this is not specific to R; ocaml suffers too.
>
> That may be (I don't know enough about gc's to really say one way or
> another), but if I remember correctly, allocation triggers gc, so
> manual triggering shouldn't be too important. In my experience, the
> one point I've needed it was after freeing multiple very large objects
> when hitting memory limits. Rewriting that code to use functions
> rather than as one long imperative slog was a real performance win.
>
> Note that if you are compiling locally, you can modify the gc
> parameters (frequency of various sweeps) for different performance
> characteristics -- grep src/main/memory.c for "Tuning Constants" and
> the lines that follow.
>
>>
>>> since a loop doesn't define its own scope like some languages (a
>>> practice that always seemed strange to me),
>>
>> every level of indentation has its own scope.
>> seems reasonable.
>
> I guess I see something like
>
> for(i in 1:2){
>    f(i)
> }
>
> as little more than short hand for
>
> f(1)
> f(2)
>
> rather than as something "more meaningful". I suppose you're thinking
> of something like Ruby blocks here? Those correspond more closely to
> anonymous functions in my mind. (scope wise)
>
> Certainly, "to each his own" applies here.

Note that according to R FAQ 3.3.3, this is actually something
intentionally different from S(+) so someone must have decided it was
important at one point. Without any reference, I'd imagine this
matches scheme, as most R scoping rules do.

http://cran.r-project.org/doc/FAQ/R-FAQ.html#R-and-S

Michael

>
>>
>>> The other answer is to use functions / apply statements like the good
>>> lord and John Chambers intended :-)
>>
>> so explicit loops are "deprecated" in some sense?
>
> No, not when they are really necessary (truly iterative computation),
> but it's generally considered clearer/idiomatic to use higher order
> functions like *apply (which I suppose is really just Map by another
> name) for brevity. The fact that *apply is, in turn, a function call
> means you get the "new scope" benefits for free. Looking forward,
> apply() statements are stateless and hence much easier to parallelize
> than loops. (In fact, the parallel package available with R >=2.14
> uses that exact abstraction)
>
> As a note, some folks worry about function calls in R: they do have
> some cost, but R keeps a copy-on-write+lazy eval behavior for function
> arguments so
>
> x <- seq_len(1e7)
> y <- 3
>
> f <- function(a,b) print(b)
>
> f(x,y)
>
> doesn't actually copy x.
>
> It's an implementation detail that I'm not sure is documented anywhere
> (I may in fact just be making it up), but it makes some folks feel
> better about using many small functions.
>
> Cheers,
> Michael
>
>>
>> thanks for your kind and informative reply!
>> --
>> Sam Steingold (http://sds.podval.org/) on Ubuntu 12.04 (precise) X 11.0.11103000
>> http://www.childpsy.net/ http://ffii.org http://mideasttruth.com
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