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 <
jeffe...@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
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--
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
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