Generate the Cartesian Product of 2 vector<string>s In-Place?

Question

If I want to get the Cartesian Product of these two vector<string>s:

vector<string> final{"a","b","c"};
vector<string> temp{"1","2"};

But I want to put the result in final, such that final would contain:

a1
a2
b1
b2
c1
c2

I'd like to do this without creating a temporary array. Is it possible to do this? If it matters, the order of final is not important.

Answer 1

You may try the following approach

#include <iostream>
#include <vector>
#include <string>

int main() 
{
    std::vector<std::string> final{ "a", "b", "c" };
    std::vector<std::string> temp{ "1", "2" };

    auto n = final.size();

    final.resize( final.size() * temp.size() );

    for ( auto i = n, j = final.size(); i != 0; --i )
    {

        for ( auto it = temp.rbegin(); it != temp.rend(); ++it )
        {
            final[--j] = final[i-1] + *it; 
        }

    }

    for ( const auto &s : final ) std::cout << s << ' ';
    std::cout << std::endl;

    return 0;
}

The program output is

a1 a2 b1 b2 c1 c2