priority_queue in C++ STL

What is Priority Queue?

In the case of the max heap, priority queues are a type of container adaptors, specifically designed such that its first element is always the greatest of the elements it contains and the rest elements are in decreasing order.

In the case of the min heap, priority queues are a type of container adaptors, specifically designed such that its first element is always the smallest of the elements it contains and the rest elements are in increasing order.

Note: In C++ STL by default max-heap is created.

The syntax for a max-heap priority queue

priority_queue<object_type> variable_name;

Example:

priority_queue<int> pq;

The syntax for a min-heap priority queue

priority_queue<object_type,vector<object_type>,greater<object_type>> variable_name;

Example:

priority_queue<int,vector<int>,greater<int>> pq;

Functions in priority queue:

push() – to insert an element in the priority queue.

pq.push(110);
pq.push(220);

pop() – deletes the top element of the priority queue.

pq.pop();

top() – returns the element at the top of the priority queue.

pq.top();

emplace() – to insert an element in the priority.

pq.emplace(1);
pq.emplace(2);

size() – returns the number of elements in the priority queue.

pq.size();

empty() – to check if the priority queue is empty or not.

pq.empty();

Code for max-heap:

C++ Code

#include<bits/stdc++.h>
using namespace std;
void printpriorityqueue(priority_queue<int> pq)
{
    priority_queue<int> pq2=pq;
    while(!pq.empty())
    {
        cout<<pq.top()<<"\n";
        pq.pop();
    }
}
int main()
{
    priority_queue<int> pq;
    for(int i=1;i<=5;i++)
    pq.push(i);
    
    cout<<"The elements of the priority queue are:"<<endl;
    printpriorityqueue(pq);
    
    cout<<"The size of the priority queue: "<<pq.size()<<endl;
    cout<<"The top element of the priority queue: "<<pq.top()<<endl;
    cout<<"Pop the top element: "<<endl;
    pq.pop();
    printpriorityqueue(pq);
}

Output:

The elements of the priority queue are:
5
4
3
2
1
The size of the priority queue: 5
The top element of the priority queue: 5
Pop the top element:
4
3
2
1

Code for min-heap:

C++ Code

#include<bits/stdc++.h>
using namespace std;
void printpriorityqueue(priority_queue<int,vector<int>,greater<int>> pq)
{
    priority_queue<int,vector<int>,greater<int>> pq2=pq;
    while(!pq.empty())
    {
        cout<<pq.top()<<"\n";
        pq.pop();
    }
}
int main()
{
    priority_queue<int,vector<int>,greater<int>> pq;
    for(int i=1;i<=5;i++)
    pq.push(i);
    
    cout<<"The elements of the priority queue are:"<<endl;
    printpriorityqueue(pq);
    
    cout<<"The size of the priority queue: "<<pq.size()<<endl;
    cout<<"The top element of the priority queue: "<<pq.top()<<endl;
    cout<<"Pop the top element: "<<endl;
    pq.pop();
    printpriorityqueue(pq);
}

Output:

The elements of the priority queue are:
1
2
3
4
5
The size of the priority queue: 5
The top element of the priority queue: 1
Pop the top element:
2
3
4
5

Special thanks to Subhrajit Das for contributing to this article on takeUforward. If you also wish to share your knowledge with the takeUforward fam, please check out this article