How do I sort a vector of pairs based on both first and second element in a pair?

Question

if i have a vector of pairs vector<pair<int, int>> arr; and passes elements like

4 5
3 7
10 5
5 7
1 5

how can i make the pair to sort the elements depend on the first and second element in a pair like this in descending order

5 7
3 7
10 5
4 5
1 5

or in ascending order

1 5
4 5 
10 5
3 7
5 7

Edit: what i want sort the vector depend on both sides for example second element 5 repeated 3 times with first elements (4, 10, 1) and second element 7 repeated 2 times with first elements (3, 5)

so if i sort them in descending order the repeated 7 will come first then 5 it will be like that

3 7
5 7
4 5
10 5
1 5

then sort the first elements with 7 also in descending order to become (5, 3) then first elements with 5 to become (10, 4, 1) so the final array will be like that

5 7
3 7
10 5
4 5
1 5

Answer 1

Use std::sort , and apply the proper sort criteria:

#include <algorithm>
#include <vector>
#include <utility>
#include <iostream>

bool SortAscending(const std::pair<int,int>& p1, const std::pair<int,int>& p2)
{
   return std::tie(p1.second, p1.first) < std::tie(p2.second, p2.first);
}

bool SortDescending(const std::pair<int,int>& p1, const std::pair<int,int>& p2)
{
   return std::tie(p1.second, p1.first) > std::tie(p2.second, p2.first);
}

int main()
{
    std::vector<std::pair<int, int>> arr = 
              {{4, 5},{3, 7}, {10, 5}, {5, 7},{1, 5}};

    std::cout << "Original: \n";
    for (auto& p : arr)
        std::cout << p.first << " " << p.second << "\n";

    std::sort(arr.begin(), arr.end(), SortAscending);
    std::cout << "\n\nAscending: \n";
    for (auto& p : arr)
        std::cout << p.first << " " << p.second << "\n";

    std::sort(arr.begin(), arr.end(), SortDescending);
    std::cout << "\n\nDescending: \n";
    for (auto& p : arr)
        std::cout << p.first << " " << p.second << "\n";
}

Output:

Original: 
4 5
3 7
10 5
5 7
1 5


Ascending: 
1 5
4 5
10 5
3 7
5 7


Descending: 
5 7
3 7
10 5
4 5
1 5

Answer 2

You can simply use the std::sort to sort it in ascending order

std::vector<std::pair<int, int>> arr;
arr.push_back({ 4, 5 });
arr.push_back({ 3, 7 });
arr.push_back({ 10, 5 });
arr.push_back({ 5, 7 });
arr.push_back({ 1, 5 });
std::sort(arr.begin(), arr.end());

In addition you can sort in descending order using std::greater<>()

std::sort(arr.begin(), arr.end(), std::greater<>());

After Edit: If your question is to sort by second element than if the second element are equal to sort by the first element than this might a solution

(it in ascending order)

bool sortBySecondElement(const std::pair<int, int>& p1, const std::pair<int, int> & p2)
{
    return p1.second < p2.second;
}

bool sortByFirstElement(const std::pair<int, int>& p1, const std::pair<int, int>& p2)
{
    return p1.second == p2.second && p1.first < p2.first;
}

int main()
{
    std::vector<std::pair<int, int>> arr;
    arr.push_back({ 4, 5 });
    arr.push_back({ 3, 7 });
    arr.push_back({ 10, 5 });
    arr.push_back({ 5, 7 });
    arr.push_back({ 1, 5 });
    std::sort(arr.begin(), arr.end(), sortBySecondElement);
    std::sort(arr.begin(), arr.end(), sortByFirstElement);
}