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

[Tomas Kalibera]

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.

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 My R is a bit outdated but the macro seems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)
> ```
>
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