[Rd] Unexpected behavior when using macro to loop over vector

[Wang Jiefei]

Thank you, Tomas. I appreciate your help. BTW, could you also add an
explicit type conversion in " ITERATE_BY_REGION_PARTIAL" macro while you
are fixing the bug? C++ compiler does not happy with the implicit
conversion from void* to T* somehow and I have to redefine it before using
the macro.

Best,
Jiefei

On Fri, Oct 25, 2019 at 11:13 AM Tomas Kalibera <tomas.kalibera using gmail.com>
wrote:

> On 10/25/19 11:01 AM, Tomas Kalibera wrote:
> > On 10/23/19 6:45 AM, Wang Jiefei wrote:
> >> Hi all,
> >>
> >> I found an unexpected behavior when I was trying to use the macro
> >> defined
> >> in "R_ext/Itermacros.h"  to loop over an atomic vector. Here is a
> >> minimum
> >> example:
> >>
> >> C++ code
> >> ```
> >> #include "R_ext/Itermacros.h"
> >> #define GET_REGION_BUFSIZE 2
> >> //Redefine the macro since C++ is not happy with the implicit type
> >> conversion
> >> #define ITERATE_BY_REGION_PARTIAL(sx, px, idx, nb, etype, vtype, \
> >>   strt, nfull, expr) do { \
> >> const etype *px = (etype*)DATAPTR_OR_NULL(sx); \
> >> if (px != NULL) { \
> >>     R_xlen_t __ibr_n__ = strt + nfull; \
> >>     R_xlen_t nb = __ibr_n__; \
> >>     for (R_xlen_t idx = strt; idx < __ibr_n__; idx += nb) { \
> >> expr \
> >>      } \
> >> } \
> >> else ITERATE_BY_REGION_PARTIAL0(sx, px, idx, nb, etype, vtype, \
> >> strt, nfull, expr); \
> >>      } while (0)
> >> // [[Rcpp::export]]
> >> void C_testPrint(SEXP x) {
> >> ITERATE_BY_REGION_PARTIAL(x, ptr, idx, nbatch, double, REAL, 1, 4, {
> >> for (R_xlen_t i = 0; i < nbatch; i++)
> >> Rprintf("idx: %lld, i: %lld, ptr:%f\n", idx, i, ptr[i]);
> >
> > You need to index "ptr" by "idx + i", not by "i". Have a look at how
> > the macros are used in R, e.g. printvector.c.
>
> Actually, the macro should do this for you, we will investigate/fix.
> Thanks for the report!
>
> Best
> Tomas
>
> >
> > Best,
> > Tomas
> >
> >> });
> >> }
> >> ```
> >>
> >> The function C_testPrint loops over its argument x and prints out one
> >> value
> >> of x at each loop. The loop starts from the second element and ends
> >> in the
> >> fifth element of x. I also redefine the buffer size to see the effect of
> >> it. Here is my R code:
> >>
> >> R code
> >> ```
> >>> C_testPrint(as.numeric(1:10))
> >> idx: 1, i: 0, ptr:2.000000
> >> idx: 1, i: 1, ptr:3.000000
> >> idx: 3, i: 0, ptr:4.000000
> >> idx: 3, i: 1, ptr:5.000000
> >>> C_testPrint(c(1,2,3,4,5,6,7,8,9,10))
> >> idx: 1, i: 0, ptr:1.000000
> >> idx: 1, i: 1, ptr:2.000000
> >> idx: 1, i: 2, ptr:3.000000
> >> idx: 1, i: 3, ptr:4.000000
> >> idx: 1, i: 4, ptr:5.000000
> >> ```
> >>
> >> There are two problems in the outputs:
> >> 1. The numbers of lines are different
> >> 2. The starting indices are not the same.
> >>
> >>  From my understanding, the first output seems correct to me. The
> >> second is
> >> not unexpected. I believe the differences are due to the
> >> accessibility of
> >> the data pointer. Did I misunderstand and misuse the macro? Or is it
> >> a bug
> >> in R? Here is my session info. My R is a bit outdated but the macro
> >> seems
> >> unchanged in R 4.0. Thanks
> >>
> >> ```
> >>> sessionInfo()
> >> R Under development (unstable) (2019-08-22 r77060)
> >> Platform: x86_64-w64-mingw32/x64 (64-bit)
> >> Running under: Windows >= 8 x64 (build 9200)
> >> ```
> >>
> >>     [[alternative HTML version deleted]]
> >>
> >> ______________________________________________
> >> R-devel using r-project.org mailing list
> >> https://stat.ethz.ch/mailman/listinfo/r-devel
> >
> >
>
>

	[[alternative HTML version deleted]]