Sorting a vector of pairs by first element in ascending order, and if equal, sort second element by descending order

Question

I have a vector of pair I want to sort. I want to sort by first element in ascending order, and if the first element is equal, I want to sort it by descending order of the second element.

Example:

When sorted, it has to be:

Is there any way I could achieve this in c++?

Use the `std::sort()` variant that accepts a custom comparison function (or function object, or lambda). — Peter, Feb 12 '22 at 12:55

score 2 · Answer 1 · answered Feb 12 '22 at 13:02

2

Like the guys in comments said, std::sort should work fine with some lambda. Something like this I guess:

std::vector<std::pair<int, int>> vec{
    {3, 2},
    {1, 9},
    {0, 4},
    {3, 3}
};

std::sort(vec.begin(), vec.end(), [](const auto& l, const auto& r){
    return (l.first == r.first) ? l.second > r.second : l.first < r.first;   
});

answered Feb 12 '22 at 13:02

Hekmatyar

335
2
10

`std::pair` has comparators. No need for a lambda. – A M Feb 12 '22 at 16:03
@ArminMontigny yes, there is a need, because the standard comparisons will not achieve the order that the OP wants. – Remy Lebeau Feb 12 '22 at 16:53
Ah. Ok. Second in descending order. I see. Thanks – A M Feb 12 '22 at 23:21

Sorting a vector of pairs by first element in ascending order, and if equal, sort second element by descending order

1 Answers1