Priority queue for user-defined types

Question

I have the below struct

struct node{
   float val;
   int count;

}

I have several objects of this struct. Now, I want to insert these objects into a priority queue of STL such that the priority queue orders the items by count. Any idea on how to do so? Preferably a min heap is preferred. I know how to do the above for primitive data types, not structs

Answer 1

Overload the < operator:

bool operator<(const node& a, const node& b) {
  return a.count > b.count;
}

I have reversed the comparison to achieve min heap wihtout passing extra arguments to the priority queue. Now you use it like this:

priority_queue<node> pq;
...

Edit: take a look at this post which seems to be almost exact duplicate: STL Priority Queue on custom class

Answer 2

#include <iostream>
#include <queue>
#include <vector>
using namespace std;
class Boxer{
    public:
        string name;
        int strength;
};
struct Comp{
    bool operator()(const Boxer& a, const Boxer& b){
        return a.strength<b.strength;
    }
};
int main(){
    Boxer boxer[3];
    boxer[0].name="uday", boxer[0].strength=23;
    boxer[1].name="manoj", boxer[1].strength=33;
    boxer[2].name="rajiv", boxer[2].strength=53;

    priority_queue< Boxer, vector<Boxer>, Comp> pq;
    pq.push(boxer[0]);
    pq.push(boxer[1]);
    pq.push(boxer[2]);
    Boxer b = pq.top();
    cout<<b.name;
    //result is Rajiv

    return 0;
}

Answer 3

1. Using greater as comparison function you can use priority queue as min heap,

   #include <bits/stdc++.h>
   
   using namespace std;
   
   int main()
   {
       priority_queue<int,vector<int>,greater<int> >pq;
       pq.push(1);
       pq.push(2);
       pq.push(3);
   
       while(!pq.empty())
       {
           int r = pq.top();
           pq.pop();
           cout << r << " ";
       }
       return 0;
   }
   

2. Inserting value by changing their sign (using minus (-) for positive number and using plus (+) for negative number we can use priority queue in reversed order.

   int main()
   {
       priority_queue<int>pq2;
       pq2.push(-1); //for +1
       pq2.push(-2); //for +2
       pq2.push(-3); //for +3
       pq2.push(4);  //for -4
   
       while(!pq2.empty())
       {
           int r = pq2.top();
           pq2.pop();
           cout << -r << " ";
       }
   
       return 0;
   }
   

3. For custom data types or classes we need a to tell priority queue a way of knowing on which order it will sort our data.

   struct compare
   {
       bool operator()(const int & a, const int & b)
       {
           return a>b;
       }
   };
   
   int main()
   {
   
       priority_queue<int,vector<int>,compare> pq;
       pq.push(1);
       pq.push(2);
       pq.push(3);
   
       while(!pq.empty())
       {
           int r = pq.top();
           pq.pop();
           cout << r << " ";
       }
   
       return 0;
   }
   

4. For custom structure or class you can use priority_queue in any order. Suppose, we want to sort people in descending order according to their salary and if tie then according to their age.

   struct people
   {
       int age,salary;
   };
   struct compare {
       bool operator()(const people & a, const people & b)
       {
           if(a.salary==b.salary)
           {
               return a.age>b.age;
           } else {
               return a.salary>b.salary;
           }
       }
   };
   
   int main()
   {
       priority_queue<people,vector<people>,compare> pq;
       people person1,person2,person3;
       person1.salary=100;
       person1.age = 50;
       person2.salary=80;
       person2.age = 40;
       person3.salary = 100;
       person3.age=40;
   
       pq.push(person1);
       pq.push(person2);
       pq.push(person3);
   
       while(!pq.empty())
       {
           people r = pq.top();
           pq.pop();
           cout << r.salary << " " << r.age << endl;
       }
   

5. Same result can be obtained by operator overloading :

   struct people
   {
       int age,salary;
   
       bool operator< (const people & p) const
       {
           if(salary==p.salary)
           {
               return age>p.age;
           } else {
               return salary>p.salary;
           }
       }
   };
   

In main function :

    priority_queue<people> pq;
    people person1,person2,person3;
    person1.salary=100;
    person1.age = 50;
    person2.salary=80;
    person2.age = 40;
    person3.salary = 100;
    person3.age=40;

    pq.push(person1);
    pq.push(person2);
    pq.push(person3);

    while(!pq.empty())
    {
        people r = pq.top();
        pq.pop();
        cout << r.salary << " " << r.age << endl;
    }

Answer 4

You need to provide operator< for that struct. Something like:

bool operator<(node const& x, node const& y) {
    return x.count < y.count;
}

Now you can use a priority queue from the standard library.

Answer 5

We can define user defined comparator class:

Code Snippet :

#include<bits/stdc++.h>
using namespace std;

struct man
{
  string name;
  int priority; 
};

class comparator
{
 public:
   bool operator()(const man& a, const man& b)
   {
        return a.priority<b.priority;
   }
};

int main()
{
   man arr[5];
   priority_queue<man, vector<man>, comparator> pq;

   for(int i=0; i<3; i++)
   {
     cin>>arr[i].name>>arr[i].priority;
     pq.push(arr[i]);
   }

   while (!pq.empty())
   {
     cout<<pq.top().name<<" "<<pq.top().priority;
     pq.pop();
     cout<<endl;
   }
   return 0;
}

Answer 6

Since C++11, you can write

auto comparer = [](const auto& a, const auto& b) {
    return a.priority < b.priority;
};
std::priority_queue<Item, std::vector<Item>, decltype(comparer)> queue(comparer);