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priority_queue::top() in C++ STL
• Difficulty Level : Medium
• Last Updated : 13 Jan, 2018

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

 `// CPP program to illustrate``// Implementation of top() function``#include ``#include ``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;``}`

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).

 `// CPP program to illustrate``// Application of top() function``#include ``#include ``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;``}`

Output:

```Prime - 2
Non Prime - 3
```
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