sorting using vector pair in c++

Question

I am using vector pair to sort where I am inserting first and second element using a loop

vect.push_back(make_pair(count[i],arr[i]));

sort(vect.begin(),vect.end());

cout<<vect[i].second<<" ";

if my counts are equal then I want to print the second element in the order they were inserted in vector,

but it is not happening.

Can someome tell me why?

Answer 1

From std::sort

Sorts the elements in the range [first, last) in ascending order. The order of equal elements is not guaranteed to be preserved.
1) Elements are compared using operator<.

and from std::pair::operator<

3) If lhs.first < rhs.first, returns true. Otherwise, if rhs.first < lhs.first, returns false. Otherwise, if lhs.second < rhs.second, returns true. Otherwise, returns false.

So effectively, sort compares count[i] , and if two of them are equal, it compares arr[i] .

---

If you want to compare the count s only and keep the original order for equal keys, you must use std::stable_sort(first, last, comp)

Sorts the elements in the range [first, last) in ascending order. The order of equal elements is guaranteed to be preserved.

with an appropriate comparison operator

std::stable_sort(vect.begin(), vect.end(),
     [](const std::pair<int, T> &x, const std::pair<int, T> &y) {
        return x.first < y.first;
     }
);