User defined Comparator

#1

// What will be the output of the following C++ code?

#include <bits/stdc++.h>
using namespace std;
typedef pair<int, int> pi;
struct Compare
{
bool operator()(pi const &a, pi const &b)
{
if (a.second < b.second)
{
return true;
}
else if (a.second > b.second)
{
return false;
}
else
{
if (a.first > b.first)
{
return true;
}
return false;
}
}
};
int main()
{
priority_queue<pi, vector, Compare> q;
q.push({1, 5});
q.push({5, 15});
q.push({7, 15});
q.push({10, 2});
q.push({1, 10});
cout << q.top().first << " " << q.top().second << endl;
return 0;
}

I am unable to understand how the comparator worked . kindly help.

#2

Your code works now https://ide.codingblocks.com/s/643402
In priority queue comparator works in reverse logic.
First elements are sorted in descending order on basis of 2nd element. Then if there is a tie among them then elements are sorted in ascending order on basis of first element here.

#3

In the compare class, it returns true when a.second < b.second which implies the sequence should have been sorted in ascending order(according to SECOND value), and when there is a tie, pair with larger FIRST value will be there in the sequence before the other . The sequence then should have been ->
({10,2},{1,5},{1,10},{7,15},{5,15}).
The top should be pointing to {10,2}.
Is it so or the other way around? if i am getting it wrong please suggest me some resource as i get so much confused in user defined comparators i.e. when to arrange in ascending and when to arrange descending.

#4

Like I said priority works in opposite way. So if you make comparator as ascending it gives descending. See the articles on it, it will become clearer.

1 Like
#5

Okay , your last line made things clear.
Thank you .

#6

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