[Rd] [External] Re: ALTREP ALTINTEGER_SUM/MIN/MAX Return Value and Behavior

[iuke-tier@ey m@iii@g oii uiow@@edu]

Call the R sum() function, either before going to C code or by calling
back into R. You may only want to do this if the vector is long enough
for e possible savings to be worth while.


On Tue, 29 Jun 2021, Sebastian Martin Krantz wrote:

> Thanks both. Is there a suggested way I can get this speedup in a package?
> Or just leave it for now?
>
> Thanks also for the clarification Bill. The issue I have with that is that
> in my C code ALTREP(x) evaluates to true even after adding and removing
> dimensions (otherwise it would be handled by the normal sum method and I’d
> be fine).

When you use a longer vector

> Also .Internal(inspect(x)) still shows the compact
> representation.

A different representation (wrapper around a compact sequence).

Best,

luke


> -Sebastian
>
> On Tue 29. Jun 2021 at 19:43, Bill Dunlap <williamwdunlap using gmail.com> wrote:
>
>> Adding the dimensions attribute takes away the altrep-ness.  Removing
>> dimensions
>> does not make it altrep.  E.g.,
>>
>>> a <- 1:10
>>> am <- a ; dim(am) <- c(2L,5L)
>>> amn <- am ; dim(amn) <- NULL
>>> .Call("is_altrep", a)
>> [1] TRUE
>>> .Call("is_altrep", am)
>> [1] FALSE
>>> .Call("is_altrep", amn)
>> [1] FALSE
>>
>> where is_altrep() is defined by the following C code:
>>
>> #include <R.h>
>> #include <Rinternals.h>
>>
>> SEXP is_altrep(SEXP x)
>> {
>>     return Rf_ScalarLogical(ALTREP(x));
>> }
>>
>>
>> -Bill
>>
>> On Tue, Jun 29, 2021 at 8:03 AM Sebastian Martin Krantz <
>> sebastian.krantz using graduateinstitute.ch> wrote:
>>
>>> Hello together, I'm working on some custom (grouped, weighted) sum, min
>>> and
>>> max functions and I want them to support the special case of plain integer
>>> sequences using ALTREP. I thereby encountered some behavior I cannot
>>> explain to myself. The head of my fsum C function looks like this (g is
>>> optional grouping vector, w is optional weights vector):
>>>
>>> SEXP fsumC(SEXP x, SEXP Rng, SEXP g, SEXP w, SEXP Rnarm) {
>>>   int l = length(x), tx = TYPEOF(x), ng = asInteger(Rng),
>>>     narm = asLogical(Rnarm), nprotect = 1, nwl = isNull(w);
>>>   if(ALTREP(x) && ng == 0 && nwl) {
>>>     switch(tx) {
>>>     case INTSXP: return ALTINTEGER_SUM(x, (Rboolean)narm);
>>>     case LGLSXP: return ALTLOGICAL_SUM(x, (Rboolean)narm);
>>>     case REALSXP: return ALTLOGICAL_SUM(x, (Rboolean)narm);
>>>     default: error("ALTREP object must be integer or real typed");
>>>     }
>>>   }
>>> // ...
>>> }
>>>
>>> when I let x <- 1:1e8, fsum(x) works fine and returns the correct value.
>>> If
>>> I now make this a matrix dim(x) <- c(1e2, 1e6) and subsequently turn this
>>> into a vector again, dim(x) <- NULL, fsum(x) gives  NULL and a warning
>>> message 'converting NULL pointer to R NULL'. For functions fmin and fmax
>>> (similarly defined using ALTINTEGER_MIN/MAX), I get this error right away
>>> e.g. fmin(1:1e8) gives NULL and warning 'converting NULL pointer to R
>>> NULL'. So what is going on here? What do these functions return? And how
>>> do
>>> I make this a robust implementation?
>>>
>>> Best regards,
>>>
>>> Sebastian Krantz
>>>
>>>         [[alternative HTML version deleted]]
>>>
>>> ______________________________________________
>>> R-devel using r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>>
>>
>
> 	[[alternative HTML version deleted]]
>
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-- 
Luke Tierney
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa                  Phone:             319-335-3386
Department of Statistics and        Fax:               319-335-3017
    Actuarial Science
241 Schaeffer Hall                  email:   luke-tierney using uiowa.edu
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