[Rd] What is the best way to loop over an ALTREP vector?

Wang Jiefei @zwj|08 @end|ng |rom gm@||@com
Mon Sep 23 21:12:39 CEST 2019

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

#define type_free_iter(sx, ptr, ind, nbatch,expr)\


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; \


       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;




On Wed, Aug 28, 2019 at 2:32 PM Wang Jiefei <szwjf08 using 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 <gabembecker using 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 <szwjf08 using 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]]
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>>> R-devel using r-project.org mailing list
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