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

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  • Difficulty Level : Basic
  • Last Updated : 23 Jun, 2022

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




// 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 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. 

CPP




// 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




// 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 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. 

CPP




// 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

Let us see the differences in a tabular form -:

 queue::empty() queue::size()
1.It is used to return whether the queue is emptyIt is used to return the number of elements in the 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 of boolean.Its return type is an integer.
5.Its complexity is constant.Its complexity is constant.

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