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

[Tomas Kalibera]

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]]
>>
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>
>