How to concatenate Lists in Rcpp - r

Question

I want to c() 2 lists in Rcpp, but I'm struggling to get the same structure as I would in R.
Here is some simple data + example:
rlist = list(a = "123")
listadd = list(typ = "fdb")
c(rlist, listadd)
which gives me this:
$a
[1] "123"
$typ
[1] "fdb"
With Rcpp I only found push_back to do more or less what I want but the structure is a bit different. I also tried to use emplace_back based on this reference but it doesnt seem to be implemented in Rcpp.
cppFunction('
List cLists(List x, List y) {
x.push_back(y);
return(x);
}')
which gives me:
cLists(rlist, listadd)
$a
[1] "123"
[[2]]
[[2]]$typ
[1] "fdb"
Based on this question I know that I could use Language("c",x,y).eval(); to use R's c() function and get the correct result, but that doesn't seem to be the right way.
So I was wondering how can I concatenate lists in Rcpp correctly?
EDIT:
Based on #Dirk's comment, I tried to create a new list and fill them with the other lists elements, but then I loose the element names.
cppFunction('
List cLists(List x, List y) {
int nsize = x.size();
int msize = y.size();
List out(nsize + msize);
for(int i = 0; i < nsize; i++) {
out[i] = x[i];
}
for(int i = 0; i < msize; i++) {
out[nsize+i] = y[i];
}
return(out);
}')
Output:
cLists(rlist, listadd)
[[1]]
[1] "123"
[[2]]
[1] "fdb"

Answer 1

The performance hit for your implementation seems to come from copying the name attribute to stl string vectors. You can avoid it like so:
library(Rcpp)
library(microbenchmark)
cppFunction('
List cLists(List x, List y) {
int nsize = x.size();
int msize = y.size();
List out(nsize + msize);
CharacterVector xnames = x.names();
CharacterVector ynames = y.names();
CharacterVector outnames(nsize + msize);
out.attr("names") = outnames;
for(int i = 0; i < nsize; i++) {
out[i] = x[i];
outnames[i] = xnames[i];
}
for(int i = 0; i < msize; i++) {
out[nsize+i] = y[i];
outnames[nsize+i] = ynames[i];
}
return(out);
}')
x <- as.list(runif(1e6)); names(x) <- sample(letters, 1e6, T)
y <- as.list(runif(1e6)); names(y) <- sample(letters, 1e6, T)
microbenchmark(cLists(x,y), c(x,y), times=3)
Unit: milliseconds
expr min lq mean median uq max neval cld
cLists(x, y) 31.70104 31.86375 32.09983 32.02646 32.29922 32.57198 3 a
c(x, y) 47.31037 53.21409 56.41159 59.11781 60.96220 62.80660 3 b
Note: by copying to std::string you're also losing possible character encoding information, whereas working with just R/Rcpp preserves.

Answer 2

This is what I came up with. The output is correct, but unfortunately it is also much less performant than the R version.
library(Rcpp)
cppFunction('
List cLists(List x, List y) {
int nsize = x.size();
int msize = y.size();
List out(nsize + msize);
CharacterVector xnames = x.names();
CharacterVector ynames = y.names();
for(int i = 0; i < nsize; i++) {
out[i] = x[i];
}
for(int i = 0; i < msize; i++) {
out[nsize+i] = y[i];
}
std::vector<std::string> z(x.size() + y.size());
std::copy(xnames.begin(), xnames.end(), z.begin());
std::copy(ynames.begin(), ynames.end(), z.begin() + x.size());
out.attr("names") = z;
return(out);
}')
Output:
cLists(rlist, listadd)
$a
[1] "123"
$typ
[1] "fdb"