How to return a vector of pairs

Question

I am trying to solve this challenge from testdome. I think my approach to the solution is not optimal (or even correct). But before changing my approach I was wondering is it possible to return a vector of pairs? if yes, how?. Below you will see the challenge, and afterwards my code. Thank you.

#include <stdexcept>
#include <iostream>
#include <vector>
#include <utility>

std::pair<int, int> findTwoSum(const std::vector<int>& list, int sum)
{
    int tracker = list.size();
    std::pair <int, int> result;
    std::vector <std::pair <int,int>> list1(100);
    for (int i = 0; i < (int)list.size(); i++){
        for (int j = ((int)list.size()-tracker); j < (int)list.size(); j++){

            if (list[i]+list[j] == sum)
                list1.push_back(std::make_pair(list[i], list[j]) );
        }
        tracker--;
    }

    return list1; // I am stuck here

    //throw std::logic_error("Waiting to be implemented");
}

#ifndef RunTests
int main()
{
    std::vector<int> list = {3, 1, 5, 7, 5, 9};
    std::pair<int, int> indices = findTwoSum(list, 10);
    std::cout << indices.first << '\n' << indices.second;
}
#endif

EDIT

Initially, this was my solution. But once I posted it on testdome, I am getting zero correct answers out of 4 categories. It is test number 5. That is why I thought, returning only one answer was wrong.

#include <stdexcept>
#include <iostream>
#include <vector>
#include <utility>

std::pair<int, int> findTwoSum(const std::vector<int>& list, int sum)
{
    int tracker = list.size();
    std::pair <int, int> result;
    std::vector <std::pair <int,int>> list1(100);
    for (int i = 0; i < (int)list.size(); i++){
        for (int j = ((int)list.size()-tracker); j < (int)list.size(); j++){
            if (list[i]+list[j] == sum)

                return result=std::make_pair(list[i], list[j]);
        }
        tracker--;
    }

    return std::make_pair(-1,-1);

    //throw std::logic_error("Waiting to be implemented");
}

#ifndef RunTests
int main()
{
    std::vector<int> list = {3, 1, 5, 7, 5, 9};
    std::pair<int, int> indices = findTwoSum(list, 10);
    std::cout << indices.first << '\n' << indices.second;
}
#endif

Answer 1

If you are using C++17, you have two options:

1. use auto return type deduction

auto create_vector_of_pairs() {
    std::vector<std::pair<int, int>> v;
    v.emplace_back(1, 2);
    v.emplace_back(3, 4);
    v.emplace_back(5, 6);
    return v;
}

2. use std::vector<std::pair<...>> as return type

std::vector<std::pair<int, int>> create_vector_of_pairs() {
    std::vector<std::pair<int, int>> v;
    v.emplace_back(1, 2);
    v.emplace_back(3, 4);
    v.emplace_back(5, 6);
    return v;
}

Then, whatever option from above you choose, in your main function you could do something like:

std::vector<std::pair<int, int>> v = create_vector_of_pairs();

or

auto v = create_vector_of_pairs();

Answer 2

Use can do this:

 std::vector<std::pair<int, int> >findTwoSum(const 
      std::vector<int>& list, int sum)
{
    int tracker = list.size();
    std::pair <int, int> result;
    std::vector <std::pair <int,int>> list1(100);
    /* ...*/
     return list1; 
  }
  int main()
 {
      std::vector<int> list = {3, 1, 5, 7, 5, 9};
      auto indices = findTwoSum(list, 10);
      std::cout << indices[0].first << '\n' << indices[0].second;
 }

Answer 3

If you want to return a vector of pairs, your function signature should look like

std::vector<std::pair<...>> f();

Your current function is just returning a single std::pair.

Answer 4

you just need to switch the function signature to

std::vector<std::pair<int, int>> findTwoSum(const std::vector<int>& list, int sum)

But in the main you are assigning it to a pair, so you need also to change this:

std::pair<int, int> indices = findTwoSum(list, 10); // from
std::vector<std::pair<int, int>> indices = findTwoSum(list, 10); // to
std::vector<std::pair<int, int>> indices { findTwoSum(list, 10) }; // but this is better because you call just one constructor instead of a default constructor and a copy constructor

Answer 5

You can simply use std::vector<std::pair<int, int>> as the return value of your function. (and not only std::pair<int, int> as you've done in your example)

So your function becomes:

std::vector<std::pair<int, int>> findTwoSum(const std::vector<int>& list, int sum) { ... }