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Showing content from https://www.geeksforgeeks.org/cpp/queueempty-queuesize-c-stl/ below:

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

Last Updated : 04 Jun, 2023

Queue is a type of container adaptor that operate in a first in first out (FIFO) type of arrangement. Elements are inserted at the back (end) and are deleted from the front.

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

queueName.empty()

• This function does not accept any parameter.

• It returns True if the list is empty, else it returns False.

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

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

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

The below C++ demonstrates the use of the empty() function in C++ STL's queue class.

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

int main()
{
    queue<int> myqueue;
    myqueue.push(1);

    // Queue becomes 1

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

The below problem statement demonstrates one of the applications of the empty() function in C++ STL's queue class.

Given a queue of integers, the task is to find the sum of all the integers.

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

Algorithm

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

Below is the implementation:

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

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

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

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

size() function is used to return the size of the list container which is the number of elements currently stored in the list container. 

queuename.size()

• This function does not accept any parameter.

• It returns the number of elements stored in the container.

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

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

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

The below C++ demonstrates the use of the size() function in C++ STL's queue class.

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

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

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

    cout << myqueue.size();

    return 0;
}

The below problem statement demonstrates one of the applications of the size() function in C++ STL's queue class.

Given a queue of integers, find the sum of all the integers.

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

Algorithm

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

Below is the implementation:

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

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

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

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

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