Not sure if you're using just C++ or Rcpp for C++ access but
https://purrple.cat/blog/2018/10/14/altrep-and-cpp/ has some tips on using C++
w/ALTREP.
> On Sep 23, 2019, at 3:17 PM, Wang Jiefei <szwj...@gmail.com> wrote:
>
> Sorry for post a lot of things, for the first part of code, I copied my C++
> iter macro by mistake(and you can see an explicit type casting). Here is
> the macro definition from R_exts/Itermacros.h
>
> #define ITERATE_BY_REGION_PARTIAL(sx, px, idx, nb, etype, vtype, \
>
> strt, nfull, expr) do { \
>
> * const** etype *px = 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)
>
>
> Best,
>
> Jiefei
>
> On Mon, Sep 23, 2019 at 3:12 PM Wang Jiefei <szwj...@gmail.com> wrote:
>
>> Hi Gabriel,
>>
>> I have tried the macro and found a small issue, it seems like the macro is
>> written in C and does an implicit type conversion(const void * to const int
>> *), see below. While it is allowed in C, C++ seems not happy with it. Is it
>> possible to add an explicit type casting so that it can be compatible with
>> both language?
>>
>>
>> #define ITERATE_BY_REGION_PARTIAL(sx, px, idx, nb, etype, vtype, \
>>
>> strt, nfull, expr) do { \
>>
>> *const etype *px = (const** 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)
>>
>>
>> Also, I notice that the element type(etype) and vector type(vtype) has
>> to be specified in the macro. Since the SEXP is the first argument in the
>> macro, it seems redundant to define etype and vtype for they have to match
>> the type of the SEXP. I'm wondering if this is intentional? Will there be a
>> type-free macro in R in the future? Here is a simple type-free macro I'm
>> using.
>>
>> #define type_free_iter(sx, ptr, ind, nbatch,expr)\
>>
>> switch(TYPEOF(sx)){\
>>
>> case INTSXP:\
>>
>> ITERATE_BY_REGION(sx, ptr, ind, nbatch, int, INTEGER, expr);\
>>
>> break; \
>>
>> case REALSXP:\
>>
>> ITERATE_BY_REGION(sx, ptr, ind, nbatch, double, REAL, expr);\
>>
>> break; \
>>
>> case LGLSXP:\
>>
>> ITERATE_BY_REGION(sx, ptr, ind, nbatch, int, LOGICAL, expr);\
>>
>> break; \
>>
>> default:\
>>
>> Rf_error("Unknow data type\n"); \
>>
>> break; \
>>
>> }
>>
>>
>>
>> // [[Rcpp::export]]
>>
>> double sillysum(SEXP x) {
>>
>> double s = 0.0;
>>
>> type_free_iter(x, ptr, ind, nbatch,
>>
>> {
>>
>> for (int i = 0; i < nbatch; i++) { s = s + ptr[i]; }
>>
>> });
>>
>> return s;
>>
>> }
>>
>>
>>
>>
>> Best,
>>
>> Jiefei
>>
>> On Wed, Aug 28, 2019 at 2:32 PM Wang Jiefei <szwj...@gmail.com> wrote:
>>
>>> Thank you, Gabriel. The loop macro is very helpful. It is also exciting
>>> to see that there are lots of changes in ALTREP in R devel version. I
>>> really appreciate your help!
>>>
>>> Best,
>>> Jiefei
>>>
>>> On Wed, Aug 28, 2019 at 7:37 AM Gabriel Becker <gabembec...@gmail.com>
>>> wrote:
>>>
>>>> Jiefei,
>>>>
>>>> I've been meaning to write up something about this so hopefully this
>>>> will be an impetus for me to actually do that, but until then, responses
>>>> inline.
>>>>
>>>>
>>>> On Tue, Aug 27, 2019, 7:22 PM Wang Jiefei <szwj...@gmail.com> wrote:
>>>>
>>>>> Hi devel team,
>>>>>
>>>>> I'm working on C/C++ level ALTREP compatibility for a package. The
>>>>> package
>>>>> previously used pointers to access the data of a SEXP, so it would not
>>>>> work
>>>>> for some ALTREP objects which do not have a pointer. I plan to rewrite
>>>>> the
>>>>> code and use functions like get_elt, get_region, and get_subset to
>>>>> access
>>>>> the values of a vector, so I have a few questions for ALTREP:
>>>>>
>>>>> 1. Since an ALTREP do not have to define all of the above
>>>>> functions(element, region, subset), is there any way to check which
>>>>> function has been defined for an ALTREP class? I did a search on
>>>>> RInternal.h and altrep.c but did not find a solution for it. If not,
>>>>> will
>>>>> it be added in the future?
>>>>>
>>>>
>>>> Element and region are guaranteed to always be defined and work (for
>>>> altrep and non-altrep INTSXP, REALSXP, LGLSXPs, etc, we currently don't
>>>> have region for STRSXP or VECSXP, I believe). If the altrep class does not
>>>> provide them then default methods will be used, which may be inefficient in
>>>> some cases but will work. Subset is currently a forward looking stub, but
>>>> once implimented, that will also be guaranteed to work for all valid ALTREP
>>>> classes.
>>>>
>>>>
>>>>>
>>>>> 2. Given the diversity of ALTREP classes, what is the best way to loop
>>>>> over
>>>>> an ALTREP object? I hope there can be an all-in-one function which can
>>>>> get
>>>>> the values from a vector as long as at least one of the above functions
>>>>> has
>>>>> been defined, so package developers would not be bothered by tons of
>>>>> `if-else` statement if they want their package to work with ALTREP.
>>>>> Since
>>>>> it seems like there is no such function exist, what could be the best
>>>>> way
>>>>> to do the loop under the current R version?
>>>>>
>>>>
>>>> The best way to loop over all SEXPs, which supports both altrep and
>>>> nonaltrep objects is, with the ITERATE_BY_REGION (which has been in R for a
>>>> number of released versions, at least since 3.5.0 I think) and the much
>>>> newer (devel only) ITERATE_BY_REGION_PARTIAL macros defined in
>>>> R_exts/Itermacros.h
>>>>
>>>> The meaning of the arguments is as follows for ITERATE_BY_REGION_PARTIAL
>>>> are as follows (ITERATE_BY_REGION is the same except no strt, and nfull).
>>>>
>>>>
>>>> - sx - C level variable name of the SEXP to iterate over
>>>> - px - variable name to use for the pointer populated with data from
>>>> a region of sx
>>>> - idx - variable name to use for the "outer", batch counter in the
>>>> for loop. This will contain the 0-indexed start position of the batch
>>>> you're currently processing
>>>> - nb - variable name to use for the current batch size. This will
>>>> always either be GET_REGION_BUFFSIZE (512), or the number of elements
>>>> remaining in the vector, whichever is smaller
>>>> - etype - element (C) type, e.g., int, double, of the data
>>>> - vtype - vector (access API) type, e.g, INTEGER, REAL
>>>> - strt - the 0-indexed position in the vector to start iterating
>>>> - nfull - the total number oif elements to iterate over from the
>>>> vector
>>>> - expr - the code to process a single batch (Which will do things to
>>>> px, typically)
>>>>
>>>>
>>>> So code to perform badly implemented not good idea summing of REALSXP
>>>> data might look like
>>>>
>>>> double sillysum(SEXP x) {
>>>>
>>>> double s = 0.0;
>>>>
>>>> ITERATE_BY_REGION(x, ptr, ind, nbatch, double, REAL,
>>>> {
>>>>
>>>> for(int i = 0; i < nbatch; i++) { s = s + ptr[i];}
>>>> })
>>>>
>>>> return s;
>>>> }
>>>>
>>>> For meatier examples of ITERATE_BY_REGION's use in practice you can grep
>>>> the R sources. I know it is used in the implementations of the various
>>>> C-level summaries (summary.c), print and formatting functions, and anyNA.
>>>>
>>>> Some things to remember
>>>>
>>>> - If you have an inner loop like the one above, your total position
>>>> in the original vector is ind + i
>>>> - ITERATE_BY_REGION always processes the whole vector, if you need
>>>> to only do part of it yo'll either need custom breaking for both inner
>>>> and
>>>> outer loopsl, or in R-devel you can use ITERATE_BY_REGION_PARTIAL
>>>> - Don't use the variants ending in 0, all they do is skip over
>>>> things that are a good idea in the case of non-altreps (and some very
>>>> specific altreps).
>>>>
>>>> Hope that helps.
>>>>
>>>> Best,
>>>> ~G
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>> Best,
>>>>> Jiefei
>>>>>
>>>>> [[alternative HTML version deleted]]
>>>>>
>>>>> ______________________________________________
>>>>> R-devel@r-project.org mailing list
>>>>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>>>>
>>>>
>
> [[alternative HTML version deleted]]
>
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