deque::operator= and deque::operator[] 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::operator=

This operator is used to assign new contents to the container by replacing the existing contents. 
It also modifies the size according to the new contents.

Syntax :

dequename1 = (dequename2)
Parameters :
Another container of the same type.
Result :
Assign the contents of the container passed as 
parameter to the container written on left side of the operator.

Examples: 

Input  :  mydeque1 = 1, 2, 3
          mydeque2 = 3, 2, 1, 4
          mydeque1 = mydeque2;
Output :  mydeque1 = 3, 2, 1, 4

Input  :  mydeque1 = 2, 6, 1, 5
          mydeque2 = 3, 2
          mydeque1 = mydeque2;
Output :  mydeque1 = 3, 2

Errors and Exceptions
1. If the containers are of different types, an error is thrown. 
2. It has a basic no exception throw guarantee otherwise.

C++

// CPP program to illustrate
// Implementation of = operator
#include <deque>
#include <iostream>
using namespace std;

int main()
{
    deque<int> mydeque1{ 1, 2, 3 };
    deque<int> mydeque2{ 3, 2, 1, 4 };
    mydeque1 = mydeque2;
    cout << "mydeque1 = ";
    for (auto it = mydeque1.begin(); it != mydeque1.end(); ++it)
        cout << ' ' << *it;
    return 0;
}

Output: 

mydeque1= 3 2 1 4

Time complexity: O(n)

Auxiliary Space: O(n)

deque::operator[]

This operator is used to reference the element present at position given inside the operator. It is similar to the at() function, the only difference is that the at() function throws an out-of-range exception when the position is not in the bounds of the size of deque, while this operator causes undefined behavior.

Syntax :

dequename[position]
Parameters :
Position of the element to be fetched.
Returns :
Direct reference to the element at the given position.

Examples: 

Input  :  mydeque = 1, 2, 3
          mydeque[2];
Output :  3

Input  :  mydeque = 3, 4, 1, 7, 3
          mydeque[3];
Output :  7

Errors and Exceptions
1. If the position is not present in the deque, it shows undefined behavior. 
2. It has a no exception throw guarantee otherwise. 

C++

// CPP program to illustrate
// Implementation of [] operator
#include <deque>
#include <iostream>
using namespace std;

int main()
{
    deque<int> mydeque;
    mydeque.push_back(3);
    mydeque.push_back(4);
    mydeque.push_back(1);
    mydeque.push_back(7);
    mydeque.push_back(3);
    cout << mydeque[3];
    return 0;
}

Output: 

7

Time complexity: O(1)

Auxiliary Space: O(n)

Application
Given a deque of integers, print all the integers present at even positions. 

Input  :1, 2, 3, 4, 5, 6, 7, 8, 9
Output :1 3 5 7 9
Explanation - 1, 3, 5, 7 and 9 are at position 0, 2, 4, 6 and 8 which are even

Algorithm
1. Run a loop till the size of the array. 
2. Check if the position is divisible by 2, if yes, print the element at that position.

C++

// CPP program to illustrate
// Application of [] operator
#include <deque>
#include <iostream>
using namespace std;

int main()
{
    deque<int> mydeque;
    mydeque.push_back(1);
    mydeque.push_back(2);
    mydeque.push_back(3);
    mydeque.push_back(4);
    mydeque.push_back(5);
    mydeque.push_back(6);
    mydeque.push_back(7);
    mydeque.push_back(8);
    mydeque.push_back(9);
    // Deque becomes 1, 2, 3, 4, 5, 6, 7, 8, 9

    for (int i = 0; i < mydeque.size(); ++i) {
        if (i % 2 == 0) {
            cout << mydeque[i];
            cout << " ";
        }
    }
    return 0;
}

Output: 

1 3 5 7 9

Time complexity: O(1)

Auxiliary Space: O(n)

Let us see the differences in a tabular form -:

	deque::operator=	deque::operator[]
1.	It is used to assign new contents to the container and replacing its current contents	It is used to return a reference to the element at position n in the deque container.
2.	Its syntax is -: deque& operator= (const deque& x);	Its syntax is -: reference operator[] (size_type n);
3.	It takes two parameters that are -: 1. A deque object of the same type  2. An Initializer List object.	It takes only one parameter that is the position of element in container.
4.	Its complexity is linear.	Its complexity is constant.
5.	It is defined in <deque> header file.	Its iterator validity does not changes.