Iterating std::vectors simultaneously using standard C++ [duplicate] - c++

Question

This question already has answers here:
What's the best way to iterate over two or more containers simultaneously
(11 answers)
Closed 4 years ago.
I'm new to C++ and hence would need some help in accomplishing a certain task. The problem is, I have to iterate over three or more vectors simultaneously, like so:
#include <vector>
#include <iostream>
#include <string>
#include <boost/range/combine.hpp>
using namespace std;
int main(int, char**) {
vector<string> questions = {"Planet", "Rocket", "Galaxy"};
vector<string> answers = {"Planet", "Saturn", "Star"};
vector<int> count = { 12, 34, 79};
vector<int> score = { 324, 956, 289};
vector<int> result;
vector<int> subscore;
string a, q;
int c, s;
for ( const string q : questions ) {
int match = 0;
for( auto tuple : boost::combine(answers, count) ) {
boost::tie(a,c) = tuple;
if( q.substr(0,2) == a.substr(0,2)) {std::cout << q.substr(0,3) << " " << a.substr(0,3) << endl; match = c; }
else cout << "No match!" << '\n';
}
if( match ) { result.push_back(match); }
else result.push_back(0); subscore.push_back(0);
This approach works but I can't use it in the framework we are using.
Maybe someone here can point me to a similar solution that does not depend on boost but is still efficient.
Many thanks!

Answer 1

You can use good old index:
auto size = std::min( answers.size(), count.size() ); // or at least assert that size is equal
for( size_t i = 0; i < size; ++i ) {
const auto &a = answers[i];
const auto c = count[i];
// .. same as before
note this way you possibly avoiding to make 2 copies of std::string per iteration - answers -> tuple -> a

Answer 2

This seems a transform so in C++ you can use std::transform... for example:
#include <vector>
#include <iostream>
#include <string>
#include <algorithm>
using namespace std;
int main(int, char**) {
vector<string> questions = {"Planet", "Rocket", "Galaxy"};
vector<string> answers = {"Planet", "Saturn", "Star"};
vector<int> count = { 12, 34, 79};
vector<int> result;
for(const auto& q : questions)
{
transform(begin(answers), end(answers), begin(count), back_inserter(result),
[&q](const auto& answer, auto count)
{
if (q.substr(0, 2) == answer.substr(0, 2))
{
std::cout << q.substr(0,3) << " " << answer.substr(0,3) << endl;
return count;
}
else
cout << "No Match!" << endl;
return 0;
});
}
}
Now the results vector holds all results. back_inserter is used in order to dynamically grow the result std::vector.