[Rd] True length - length(unclass(x)) - without having to call unclass()?

[Dénes Tóth]

Hi Tomas,

On 09/03/2018 11:49 AM, Tomas Kalibera wrote:
> Please don't do this to get the underlying vector length (or to achieve 
> anything else). Setting/deleting attributes of an R object without 
> checking the reference count violates R semantics, which in turn can 
> have unpredictable results on R programs (essentially undebuggable 
> segfaults now or more likely later when new optimizations or features 
> are added to the language). Setting attributes on objects with reference 
> count (currently NAMED value) greater than 0 (in some special cases 1 is 
> ok) is cheating - please see Writing R Extensions - and getting speedups 
> via cheating leads to fragile, unmaintainable and buggy code. 

Please note that data.table::setattr is an exported function of a widely 
used package (available from CRAN), which also has a description in 
?data.table::setattr why it might be useful.

Of course one has to use set* functions from data.table with extreme 
care, but if one does it in the right way, they can help a lot. For 
example there is no real danger of using them in internal functions 
where one can control what is get passed to the function or created 
within the function (so when one knows that the refcount==0 condition is 
true).

(Notwithstanding the above, but also supporting you argumentation, it 
took me hours to debug a particular problem in one of my internal 
packages, see https://github.com/Rdatatable/data.table/issues/1281)

In the present case, an important and unanswered question is (cited from 
Henrik):
 >>> However, I'm concerned that calling unclass(x) may trigger an
 >>> expensive copy internally in some cases.  Is that concern unfounded?

If no copy is made, length(unclass(x)) beats length(setattr(..)) in all 
scenarios.


> Doing so 
> in packages is particularly unhelpful to the whole community - packages 
> should only use the public API as documented.
> 
> Similarly, getting a physical address of an object to hack around 
> whether R has copied it or not should certainly not be done in packages 
> and R code should never be working with or even obtaining physical 
> address of an object. This is also why one cannot obtain such address 
> using base R (apart in textual form from certain diagnostic messages 
> where it can indeed be useful for low-level debugging).

Getting the physical address of the object was done exclusively for 
demonstration purposes. I totally agree that is should not be used for 
the purpose you described and I have never ever done so.

Regards,
Denes

> 
> Tomas
> 
> On 09/02/2018 01:19 AM, Dénes Tóth wrote:
>> The solution below introduces a dependency on data.table, but 
>> otherwise it does what you need:
>>
>> ---
>>
>> # special method for Foo objects
>> length.Foo <- function(x) {
>>   length(unlist(x, recursive = TRUE, use.names = FALSE))
>> }
>>
>> # an instance of a Foo object
>> x <- structure(list(a = 1, b = list(b1 = 1, b2 = 2)), class = "Foo")
>>
>> # its length
>> stopifnot(length(x) == 3L)
>>
>> # get its length as if it were a standard list
>> .length <- function(x) {
>>   cls <- class(x)
>>   # setattr() does not make a copy, but modifies by reference
>>   data.table::setattr(x, "class", NULL)
>>   # get the length
>>   len <- base::length(x)
>>   # re-set original classes
>>   data.table::setattr(x, "class", cls)
>>   # return the unclassed length
>>   len
>> }
>>
>> # to check that we do not make unwanted changes
>> orig_class <- class(x)
>>
>> # check that the address in RAM does not change
>> a1 <- data.table::address(x)
>>
>> # 'unclassed' length
>> stopifnot(.length(x) == 2L)
>>
>> # check that address is the same
>> stopifnot(a1 == data.table::address(x))
>>
>> # check against original class
>> stopifnot(identical(orig_class, class(x)))
>>
>> ---
>>
>>
>> On 08/24/2018 07:55 PM, Henrik Bengtsson wrote:
>>> Is there a low-level function that returns the length of an object 'x'
>>> - the length that for instance .subset(x) and .subset2(x) see? An
>>> obvious candidate would be to use:
>>>
>>> .length <- function(x) length(unclass(x))
>>>
>>> However, I'm concerned that calling unclass(x) may trigger an
>>> expensive copy internally in some cases.  Is that concern unfounded?
>>>
>>> Thxs,
>>>
>>> Henrik
>>>
>>> ______________________________________________
>>> R-devel using r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>>
>>
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> 
> 
>