Sorting vector based on tuple value - c++

Question

I have below data structure,
typedef vector< tuple<int,int,int> > vector_tuple;
In vector i am storing tuple<value,count,position>
I want to sort my vector based on count, If count is same then based on position sort the vector.
structure ordering
{
bool ordering()(....)
{
return /// ?
}
};
int main()
{
std::vector<int> v1{1,1,1,6,6,5,4,4,5,5,5};
std::vector<int> v2(v1);
vector_tuple vt;
std::tuple<int,int,int> t1;
std::vector<int>::iterator iter;
int sizev=v1.size();
for(int i=0; i < sizev ; i++)
{
auto countnu = count(begin(v2),end(v2),v1[i]);
if(countnu > 0)
{
v2.erase(std::remove(begin(v2),end(v2),v1[i]),end(v2));
auto t = std::make_tuple(v1[i], countnu, i);
vt.push_back(t);
}
}
sort(begin(vt),end(vt),ordering(get<1>vt); // I need to sort based on count and if count is same, sort based on position.
for (int i=0; i < vt.size(); i++)
{
cout << get<0>(vt[i]) << " " ;
cout << get<1>(vt[i]) << " " ;
cout << get<2>(vt[i]) << " \n" ;
}
}

Answer 1

Your compare method should look like:
auto ordering = [](const std::tuple<int, int, int>& lhs,
const std::tuple<int, int, int>& rhs) {
return std::tie(std::get<1>(lhs), std::get<2>(lhs))
< std::tie(std::get<1>(rhs), std::get<2>(rhs));
};
std::sort(std::begin(vt), std::end(vt), ordering);

Answer 2

All credit to Jarod42 for the std::tie answer.
Now let's make it generic by creating a variadic template predicate:
template<std::size_t...Is>
struct tuple_parts_ascending
{
template<class...Ts>
static auto sort_order(const std::tuple<Ts...>& t)
{
return std::tie(std::get<Is>(t)...);
}
template<class...Ts>
bool operator()(const std::tuple<Ts...>& l,
const std::tuple<Ts...>& r) const
{
return sort_order(l) < sort_order(r);
}
};
which we can invoke thus:
sort(begin(vt),end(vt),tuple_parts_ascending<1,2>());
Full Code:
#include <vector>
#include <algorithm>
#include <tuple>
#include <iostream>
using namespace std;
typedef std::vector< std::tuple<int,int,int> > vector_tuple;
template<std::size_t...Is>
struct tuple_parts_ascending
{
template<class...Ts>
static auto sort_order(const std::tuple<Ts...>& t)
{
return std::tie(std::get<Is>(t)...);
}
template<class...Ts>
bool operator()(const std::tuple<Ts...>& l,
const std::tuple<Ts...>& r) const
{
return sort_order(l) < sort_order(r);
}
};
int main()
{
std::vector<int> v1{1,1,1,6,6,5,4,4,5,5,5};
std::vector<int> v2(v1);
vector_tuple vt;
std::tuple<int,int,int> t1;
std::vector<int>::iterator iter;
int sizev=v1.size();
for(int i=0; i < sizev ; i++)
{
auto countnu = count(begin(v2),end(v2),v1[i]);
if(countnu > 0)
{
v2.erase(std::remove(begin(v2),end(v2),v1[i]),end(v2));
auto t = std::make_tuple(v1[i], countnu, i);
vt.push_back(t);
}
}
sort(begin(vt),end(vt),tuple_parts_ascending<1,2>());
for (int i=0; i < vt.size(); i++)
{
cout << get<0>(vt[i]) << " " ;
cout << get<1>(vt[i]) << " " ;
cout << get<2>(vt[i]) << " \n" ;
}
}
Expected results:
6 2 3
4 2 6
1 3 0
5 4 5
Going further, we could make this operation a little more generic and 'library-worthy' by allowing the ordering predicate and the indices to be passed as parameters (this solution required c++14):
namespace detail {
template<class Pred, std::size_t...Is>
struct order_by_parts
{
constexpr
order_by_parts(Pred&& pred)
: _pred(std::move(pred))
{}
template<class...Ts>
constexpr
static auto sort_order(const std::tuple<Ts...>& t)
{
return std::tie(std::get<Is>(t)...);
}
template<class...Ts>
constexpr
bool operator()(const std::tuple<Ts...>& l,
const std::tuple<Ts...>& r) const
{
return _pred(sort_order(l), sort_order(r));
}
private:
Pred _pred;
};
}
template<class Pred, size_t...Is>
constexpr
auto order_by_parts(Pred&& pred, std::index_sequence<Is...>)
{
using pred_type = std::decay_t<Pred>;
using functor_type = detail::order_by_parts<pred_type, Is...>;
return functor_type(std::forward<Pred>(pred));
}
Now we can sort like so:
sort(begin(vt),end(vt),
order_by_parts(std::less<>(), // use predicate less<void>
std::index_sequence<1, 2>())); // pack indices into a sequence

Answer 3

Using lambdas:
std::sort(std::begin(vt),std::end(vt),[](const auto& l,const auto& r){
if(std::get<1>(l)== std::get<1>(r)){
return std::get<2>(l) < std::get<2>(r);
}
return std::get<1>(l) < std::get<1>(r);
});