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

[Sebastian Martin Krantz]

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

-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
>>
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>>
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>

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