R devel - Oct 2019 - Unexpected behavior when using macro to loop over vector

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]);
});
}
```

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 My R is a bit outdated but the macro seems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]]

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)
> ```
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel

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 at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-devel
>
>