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

Deque or Double ended queues are sequence containers with the feature of expansion and contraction on both the ends. They are similar to vectors, but are more efficient in case of insertion and deletion of elements at the end, and also the beginning. Unlike vectors, contiguous storage allocation may not be guaranteed.

deque::empty()

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

Syntax :

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

Examples:

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

Errors and Exceptions

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

// CPP program to illustrate 
// Implementation of empty() function 
#include <deque> 
#include <iostream> 
using namespace std; 
  
int main() 
{ 
    deque<int> mydeque; 
    mydeque.push_front(1); 
  
    // Deque becomes 1 
  
    if (mydeque.empty()) { 
        cout << "True"; 
    } 
    else { 
        cout << "False"; 
    } 
    return 0; 
} 

Output:

False

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

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

Algorithm
1. Check if the deque 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 deque is empty.
3. Print the final value of the variable.

// CPP program to illustrate 
// Application of empty() function 
#include <deque> 
#include <iostream> 
using namespace std; 
  
int main() 
{ 
    int sum = 0; 
    deque<int> mydeque; 
    mydeque.push_back(1); 
    mydeque.push_back(8); 
    mydeque.push_back(3); 
    mydeque.push_back(6); 
    mydeque.push_back(2); 
  
    // deque becomes 1, 8, 3, 6, 2 
  
    while (!mydeque.empty()) { 
        sum = sum + mydeque.front(); 
        mydeque.pop_front(); 
    } 
    cout << sum; 
    return 0; 
} 

Output:

deque::size()

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

Syntax :

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

Examples:

Input :   mydeque = 0, 1, 2
          mydeque.size();
Output :  3
 
Input :   mydeque = 0, 1, 2, 3, 4, 5
          mydeque.size();
Output :  6

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 <deque> 
#include <iostream> 
using namespace std; 
  
int main() 
{ 
    int sum = 0; 
    deque<int> mydeque; 
    mydeque.push_back(1); 
    mydeque.push_back(8); 
    mydeque.push_back(3); 
    mydeque.push_back(6); 
    mydeque.push_back(2); 
  
    // Deque becomes 1, 8, 3, 6, 2 
  
    cout << mydeque.size(); 
  
    return 0; 
} 

Output:

Application :
Given a deque 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 deque 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 deque size becomes 0.
3. Print the final value of the variable.

// CPP program to illustrate 
// Application of size() function 
#include <deque> 
#include <iostream> 
using namespace std; 
  
int main() 
{ 
    int sum = 0; 
    deque<int> mydeque; 
    mydeque.push_back(1); 
    mydeque.push_back(8); 
    mydeque.push_back(3); 
    mydeque.push_back(6); 
    mydeque.push_back(2); 
  
    // Deque becomes 1, 8, 3, 6, 2 
  
    while (mydeque.size() > 0) { 
        sum = sum + mydeque.front(); 
        mydeque.pop_front(); 
    } 
    cout << sum; 
    return 0; 
} 

Output:

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