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

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

It depends on the size. For a larger vector adding dim will create a
wrapper ALTREP.

Currently the wrapper does not try to use the payload's sum method;
this could be added.

Best,

luke

On Tue, 29 Jun 2021, Bill Dunlap 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]]
>>
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>
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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
Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu