Cannot sort vector<pair<int, int>> - c++

Question

I tried to sort a vector<pair<int, int>> by using STL sort with my own boolean function.
#include <bits/stdc++.h>
using namespace std;
bool comp(pair<int, int> u1, pair<int, int> u2){
if(u1.first != u2.first) return u1.first < u2.first;
return u2.second > u2.second;
}
int main(void){
ios_base::sync_with_stdio(false);
cin.tie(NULL); cout.tie(NULL);
int univs, day, pay, max_income = 0, spent = 0;
vector<pair<int, int>> day_pay;
cin >> univs;
for(int u = 0; u < univs; u++){
cin >> pay >> day;
day_pay.push_back(make_pair(day, pay));
}
for(int i = 0; i < univs; i++) cout << day_pay[i].first << " " << day_pay[i].second << endl;
cout << endl;
sort(day_pay.begin(), day_pay.end(), comp);
for(int i = 0; i < univs; i++) cout << day_pay[i].first << " " << day_pay[i].second << endl;
for(int u = 0; u < univs; u++){
if(day_pay[u].first <= spent) continue;
max_income += day_pay[u].second;
spent++;
}
cout << max_income;
}
this is test case:
4
50 2
10 1
20 2
30 1
I want to sort this case as
30 1
10 1
50 2
20 2
What should I do for solving this problem?

Answer 1

The line
return u2.second > u2.second;
is suspicious. You most likely mean
return u1.second > u2.second;

Answer 2

Look at your comp function. The line after the if:
return u2.second > u2.second;
shuldn't that be:
return u1.second > u2.second

Answer 3

The actual fault was well pointed by Caleth in their answer. But I think that this much code is sufficient for sorting. You may modify it according to your question.
#include <iostream>
#include <utility>
#include <vector>
int main() {
std::size_t n;
std::cin >> n;
std::vector<std::pair<int, int>> v(n);
for (auto &&i : v) std::cin >> i.first >> i.second;
std::sort(v.begin(), v.end(),
[](std::pair<int, int> &a, std::pair<int, int> &b) {
return a.second != b.second ? a.second < b.second
: a.first > b.first;
});
for (auto &&i : v) std::cout << i.first << ' ' << i.second << '\n';
}
If you're using C++ 14 or higher, you can directly use auto in lambda params.
Sample Run:
Sample Input:
4
50 2
10 1
20 2
30 1
Corresponding Output:
30 1
10 1
50 2
20 2