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

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

queue::empty()

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

Syntax :

queuename.empty()
Parameters :
No parameters are passed
Returns :
True, if list is empty
False, Otherwise

Examples:

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

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

Errors and Exceptions



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

Output:

False

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

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

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

// 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;
}

Output:

20
queue::size()

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

Syntax :

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

Examples:

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

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

Errors and Exceptions

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

Output:

5

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

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

Algorithm
1. Check if the size of the queue is zero, if not add the front element to a variable initialised 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.

// 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;
}

Output:

20




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