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

[Tomas Kalibera]

On 10/25/19 5:20 PM, Wang Jiefei wrote:
> 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.

Yes, C++ requires a cast from void to non-void pointer. Fixed now in 
R-devel (77331): your example iterator should now work also for the 
non-compact sequence.

Best
Tomas

>
> Best,
> Jiefei
>
> On Fri, Oct 25, 2019 at 11:13 AM Tomas Kalibera 
> <tomas.kalibera using gmail.com <mailto: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 <mailto:R-devel using r-project.org> mailing list
>     >> https://stat.ethz.ch/mailman/listinfo/r-devel
>     >
>     >
>


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