priority_queue::empty() and priority_queue::size() in C++ STL

Priority queues are a type of container adaptors, specifically designed such that the first element of the queue is either the greatest or the smallest of all elements in the queue. However, in C++ STL (by default) the largest element is at the top. We can also create a priority queue having the smallest element at the top by simply passing an extra parameter while creating the priority queue.

priority_queue::empty()

empty() function is used to check if the priority queue container is empty or not. 

Syntax :

Time complexity: O(1)

pqueuename.empty()
Parameters :
No parameters are passed
Returns :
True, if priority queue is empty, 
False, Otherwise

Examples:

Input :  pqueue = 3, 2, 1
         pqueue.empty();
Output : False

Input :  pqueue
         pqueue.empty();
Output : True

Errors and Exceptions

1. Shows error if a parameter is passed 
2. Shows no exception throw guarantee. 

CPP

// CPP program to illustrate
// Implementation of empty() function
#include <iostream>
#include <queue>
using namespace std;

int main()
{
    priority_queue<int> pqueue;
    pqueue.push(1);

    // Priority Queue becomes 1

    if (pqueue.empty()) {
        cout << "True";
    }
    else {
        cout << "False";
    }
    return 0;
}

Output:

False

Application : Given a priority queue of integers, find the sum of the all the integers.

Input  : 8, 6, 3, 2, 1 
Output : 20

Algorithm

1. Check if the priority queue is empty, if not add the top element to a variable initialised as 0, and pop the top element. 
2. Repeat this step until the priority queue is empty. 
3. Print the final value of the variable. 

CPP

// CPP program to illustrate
// Application of empty() function
#include <iostream>
#include <queue>
using namespace std;

int main()
{
    int sum = 0;
    priority_queue<int> pqueue;
    pqueue.push(8);
    pqueue.push(6);
    pqueue.push(3);
    pqueue.push(2);
    pqueue.push(1);

    // Queue becomes 8, 6, 3, 2, 1

    while (!pqueue.empty()) {
        sum = sum + pqueue.top();
        pqueue.pop();
    }
    cout << sum;
    return 0;
}

Output:

20

priority_queue::size()

size() function is used to return the size of the priority queue container or the number of elements in the container.

Time complexity: O(1)

Syntax :

pqueuename.size()
Parameters :
No parameters are passed
Returns :
Number of elements in the container

Examples:

Input :  pqueue = 3, 2, 1
         pqueue.size();
Output : 3

Input :  pqueue
         pqueue.size();
Output : 0

Errors and Exceptions

1. Shows error if a parameter is passed. 
2. Shows no exception throw guarantee 

CPP

// CPP program to illustrate
// Implementation of size() function
#include <iostream>
#include <queue>
using namespace std;

int main()
{
    int sum = 0;
    priority_queue<int> pqueue;
    pqueue.push(8);
    pqueue.push(6);
    pqueue.push(3);
    pqueue.push(3);
    pqueue.push(1);

    // Priority Queue becomes 8, 6, 3, 2, 1

    cout << pqueue.size();

    return 0;
}

Output:

5

Application : Given a priority queue of integers, find the sum of the all the integers.

Input  : 8, 6, 3, 2, 1 
Output : 20

Algorithm

1. Check if the size of the priority queue is 0, if not add the top element to a variable initialised as 0, and pop the top element. 
2. Repeat this step until the sizeof the priority queue becomes 0. 
3. Print the final value of the variable. 

CPP

// CPP program to illustrate
// Application of size() function
#include <iostream>
#include <queue>
using namespace std;

int main()
{
    int sum = 0;
    priority_queue<int> pqueue;
    pqueue.push(8);
    pqueue.push(6);
    pqueue.push(3);
    pqueue.push(2);
    pqueue.push(1);

    // Queue becomes 8, 6, 3, 2, 1

    while (pqueue.size() > 0) {
        sum = sum + pqueue.top();
        pqueue.pop();
    }
    cout << sum;
    return 0;
}

Output:

20

Let us see the differences in a tabular form -:

	priority_queue::empty()	priority_queue::size()
1.	It is used to check whether the priority_queue is empty	It is used to return the number of elements in the priority_queue.
2.	Its syntax is -:  empty();	Its syntax is -: size();
3.	It does not take any parameters.	It does not take any parameters.
4.	Its return type is boolean.	Its return type is an integer.
5.	Its complexity is constant.	Its complexity is constant.