Converting R split() function to C++ - c++

Question

Consider the reproducible example in R:
test <- c(1:12)
> test
[1] 1 2 3 4 5 6 7 8 9 10 11 12
The expected result:
test.list <- split(test, gl(2, 3))
> test.list
$`1`
[1] 1 2 3 7 8 9
$`2`
[1] 4 5 6 10 11 12
I am trying to write equivalent code in C++ to produce and return the two vectors that resulted from the test.list. Note that, I am in the embarrassing novice stage in C++.

Answer 1

We can use the nice answer by #jignatius and make it an R-callable function. For simplicity I keep it at NumericVector; we have a boatload of answers here that show show to switch between NumericVector and IntegerVector based on the run-time payload.
Code
#include <Rcpp.h>
// [[Rcpp::export]]
Rcpp::List mysplit(Rcpp::NumericVector nums, int n, int size) {
std::vector<std::vector<double>> result(n);
int i = 0;
auto beg = nums.cbegin();
auto end = nums.cend();
while (beg != nums.cend()) {
//get end iterator safely
auto next = std::distance(beg, end) >= size ? beg + size : end;
//insert into result
result[i].insert(result[i].end(), beg, next);
//advance iterator
beg = next;
i = (i + 1) % n;
}
Rcpp::List ll;
for (const auto&v : result)
ll.push_back(v);
return ll;
}
/*** R
testvec <- 1:12
mysplit(testvec, 2, 3)
*/
Output
> Rcpp::sourceCpp("~/git/stackoverflow/68858728/answer.cpp")
> testvec <- 1:12
> mysplit(testvec, 2, 3)
[[1]]
[1] 1 2 3 7 8 9
[[2]]
[1] 4 5 6 10 11 12
>
There is a minor error in the original question in that we do not need a call to gl(); just the two scalars are needed.

Answer 2

Try this, which creates a vector of vectors containing the elements from the source in alternating chunks:
#include <iostream>
#include <vector>
template<typename T>
std::vector<std::vector<T>> split(std::vector<T> nums, int n, int size)
{
std::vector<std::vector<T>> result(n);
int i = 0;
auto beg = nums.cbegin();
auto end = nums.cend();
while (beg != nums.cend()) {
//get end iterator safely
auto next = std::distance(beg, end) >= size ? beg + size : end;
//insert into result
result[i].insert(result[i].end(), beg, next);
//advance iterator
beg = next;
i = (i + 1) % n;
}
return result;
}
int main()
{
std::vector<int> vnums = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 };
auto vectors = split(vnums, 2, 3);
for (const auto& v : vectors)
{
for (auto num : v) {
std::cout << num << " ";
}
std::cout << std::endl;
}
}
Demo