Question

Very simple question: is there a smart way of creating a subvector from regularly spaced elements of another vector with the STL?

In short, is it possible to write the following code with a STL algorithm:

int inc = 2;
std::vector<double> v_origin;
std::vector<double> v_dest;

for (int i = 0; i < v_origin.size(); i+= inc)
    v_dest.push_back(v_origin[i]);

Like I would write in Matlab or Python something like:

v_dest = v_origin[0:inc:end];

Answer 1

As a general solution, you could define a stride iterator. If you use Boost.Range, then it already as a strided range adaptor.

Example:

#include <vector>
#include <iostream>
#include <boost/range/adaptors.hpp>
#include <boost/range/algorithm.hpp>

int main()
{
    int inc = 2;
    std::vector<double> v_origin;
    std::vector<double> v_dest;

    for (int i = 0; i < 10; ++ i)
        v_origin.push_back(i);

    boost::copy(v_origin | boost::adaptors::strided(2),
                std::back_inserter(v_dest));
    // ^ In Python:    v_dest[] = v_origin[::2]

    boost::copy(v_dest, std::ostream_iterator<double>(std::cout, ", "));
}