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

[Gabriel Becker]

Hi Sebastian,

So the way that it is currently factored, there isn't a good way of getting
what you want under the constraints of what Luke said (ALTINTEGER_SUM is
not part of the API).

I don't know what his reason are for saying that per say and would not want
to speak for him, but of the top of my head, I suspect it is because ALTREP
sum methods are allowed to return NULL (the C version) to say "I don't have
a sum method that is applicable here, please continue with the normal
code". So, just as an example, your exact code is likely to segfault, I
think, if you hit an ALTREP that chooses not to implement a sum method
because you'll be running around with a SEXP that has the value NULL (the C
one, not the R one).

One thing you could do, is check for altrepness and then construct and
evaluate a call to the R sum function in that case, but that probably isn't
quite what you want either, as this will hit the code you're trying to
bypass/speedup  in the case where the ALTREP class doesn't implement a sum
methods. I see that Luke just mentioned this as well but I'll leave it in
since I had already typed it.

I hope that helps clarify some things.

Best,
~G


On Tue, Jun 29, 2021 at 10:13 AM Sebastian Martin Krantz <
sebastian.krantz using graduateinstitute.ch> 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). 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
> >>
> >>         [[alternative HTML version deleted]]
> >>
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> >>
> >
>
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>
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