[Rcpp-devel] Rcpp equivalent of sapply(L, length)

[Dr Gregory Jefferis]

Dear Qiang,

Thank you - that will do perfectly!

And thanks also to Simon Urbanek for pointing out the R `lengths()` 
function which does the same on the R side.

What great help in so little time. Thank you, colleagues!

Greg.

On 2 Jul 2021, at 2:39, Qiang Kou wrote:

> What about using "Rf_length"?
>
> -------------------------------------------------------
> #include <Rcpp.h>
>
> using namespace Rcpp;
>
> // [[Rcpp::export]]
> IntegerVector c_listlengths(List L) {
>     IntegerVector lens(L.size());
>
>     for (int i=0; i<L.size(); i++) {
>         SEXP x = L[i];
>         lens(i) = Rf_length(x);
>     }
>     return lens;
> }
> ----------------------------------------------------------
>
> In R:
>
>> Rcpp::sourceCpp("example.cpp")
>> l=list(1:3, 2:3, 1:6)
>> l2=list(letters[1:3], letters[2:3], letters[1:6])
>> c_listlengths(l)
> [1] 3 2 6
>> c_listlengths(l2)
> [1] 3 2 6
>> l3=list(letters[1:6], letters[2:3], letters[1:3])
>> c_listlengths(l3)
> [1] 6 2 3
>
>
> On Thu, Jul 1, 2021 at 5:14 PM Dr Gregory Jefferis <
> jefferis at mrc-lmb.cam.ac.uk> wrote:
>
>> Dear RcppUsers,
>>
>> I feel like there must be a simple way to do the equivalent of
>>
>> sapply(L, length)
>>
>> in Rcpp but I can't seem to get past the problem of converting the
>> elements of L into a type for which .size() or .length() are valid. I
>> see that I could do this using some long switch(TYPEOF(x)) statement 
>> (eg
>> https://gallery.rcpp.org/articles/rcpp-return-macros/):
>>
>> // [[Rcpp::export]]
>> IntegerVector c_listlengths(List L) {
>>    IntegerVector lens(L.size());
>>
>>    for (int i=0; i<L.size(); i++) {
>>      SEXP x = L[i];
>>      switch (TYPEOF(x)) {
>>      case INTSXP: {
>>        lens(i)=as<IntegerVector>(x).size();
>>      }
>>         // handle other SXP types
>>      }
>>    }
>>    return lens;
>> }
>>
>> but that that seems horribly convoluted. Can anyone point me to a
>> simpler way?
>>
>> Apologies if this is an FAQ, but I did not yet manage to turn up
>> anything close enough.
>>
>> With many thanks,
>>
>> Greg.
>>
>> # R example
>> l=list(1:3, 2:3, 1:6)
>> l2=list(letters[1:3], letters[2:3], letters[1:6])
>> sapply(l, length)
>> sapply(l2, length)
>>
>>
>>
>> --
>> Gregory Jefferis
>> Division of Neurobiology
>> MRC Laboratory of Molecular Biology
>> Francis Crick Avenue
>> Cambridge Biomedical Campus
>> Cambridge, CB2 OQH, UK
>>
>> http://www2.mrc-lmb.cam.ac.uk/group-leaders/h-to-m/g-jefferis
>> http://jefferislab.org
>> https://www.zoo.cam.ac.uk/research/groups/connectomics
>> _______________________________________________
>> Rcpp-devel mailing list
>> Rcpp-devel at lists.r-forge.r-project.org
>> https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel
>
> -- 
>  <https://nyquistdata.com/>  
> <https://www.linkedin.com/company/nyquistdata/>  
> <https://twitter.com/NyquistData>


--
Gregory Jefferis, PhD                   Tel: +44 1223 267048
Division of Neurobiology
MRC Laboratory of Molecular Biology
Francis Crick Avenue
Cambridge Biomedical Campus
Cambridge, CB2 0QH, UK

http://www2.mrc-lmb.cam.ac.uk/group-leaders/h-to-m/g-jefferis
http://jefferislab.org
http://www.zoo.cam.ac.uk/research/groups/connectomics