priority_queue::top() in C++ STL


Priority queues are a type of container adaptors, specifically designed such that the first element of the queue is the greatest of all elements in the queue.

priority_queue::top()

top() function is used to reference the top(or the largest) element of the priority queue.
Syntax :

pqueuename.top()
Parameters :
No value is needed to pass as the parameter.
Returns :
Direct reference to the top(or the largest)
element of the priority queue container.

Examples:

Input  : pqueue.push(5);
         pqueue.push(1);
         pqueue.top();
Output : 5

Input  : pqueue.push(5);
         pqueue.push(1);
         pqueue.push(7);
         pqueue.top();
Output : 7

Errors and Exceptions

1. If the priority queue container is empty, it causes undefined behaviour
2. It has a no exception throw guarantee if the priority queue is not empty

filter_none

edit
close

play_arrow

link
brightness_4
code

// CPP program to illustrate
// Implementation of top() function
#include <iostream>
#include <queue>
using namespace std;
  
int main()
{
    priority_queue<int> pqueue;
    pqueue.push(5);
    pqueue.push(1);
    pqueue.push(7);
  
    // Priority queue top
    cout << pqueue.top();
    return 0;
}

chevron_right


Output:

7

Application :
Given a priority queue of integers, find the number of prime and non prime numbers.

Input : 8, 6, 3, 2, 1
Output: Prime - 2
        Non Prime - 3

Algorithm
1. Enter the size of the priority queue into a variable.
2. Check if the priority queue is empty, check if the top element is prime, if prime increments the prime counter, and pop the top element.
3. Repeat this step until the priority queue is empty.
4. Print the final value of the variable prime and nonprime(size – prime).

filter_none

edit
close

play_arrow

link
brightness_4
code

// CPP program to illustrate
// Application of top() function
#include <iostream>
#include <queue>
using namespace std;
  
int main()
{
    int prime = 0, nonprime = 0, size;
    priority_queue<int> pqueue;
    pqueue.push(1);
    pqueue.push(8);
    pqueue.push(3);
    pqueue.push(6);
    pqueue.push(2);
    size = pqueue.size();
  
    // Priority queue becomes 1, 8, 3, 6, 2
  
    while (!pqueue.empty()) {
        for (int i = 2; i <= pqueue.top() / 2; ++i) {
            if (pqueue.top() % i == 0) {
                prime++;
                break;
            }
        }
        pqueue.pop();
    }
    cout << "Prime - " << prime << endl;
    cout << "Non Prime - " << size - prime;
    return 0;
}

chevron_right


Output:

Prime - 2
Non Prime - 3


My Personal Notes arrow_drop_up

Check out this Author's contributed articles.

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.