Converting R split() function to 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.