# deque::at() and deque::swap() in C++ programming STL

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In this article we will be discussing the working, syntax and examples of deque::at() and deque::swap() functions in C++ STL.

## What is Deque?

Deque is the Double Ended Queues that are the sequence containers which provides the functionality of expansion and contraction on both the ends. A queue data structure allow user to insert data only at the END and delete data from the FRONT. Let’s take the analogy of queues at bus stops where the person can be inserted to a queue from the END only and the person standing in the FRONT is the first to be removed whereas in Double ended queue the insertion and deletion of data is possible at both the ends.

## What is deque::at()?

deque::at() is an inbuilt function in C++ STL which is declared in <deque> header file. deque::at() returns a reference that is used to point the element present at the specified position in the deque. This function goes to the element present at the position passed as an argument to the function. The position value starts from 0.

## Syntax

mydeque.at(int position);

## Parameter

This function accepts one parameter for the position where we want to point.

## Return value

It returns a reference to the element on the specified position of the deque container.

## Example

Input: deque<int> mydeque = {10, 20, 30, 40};
mydeque.at(2);

## Output

Element at 2 positions is 30.

## Example

Live Demo

#include <deque>
#include <iostream>
using namespace std;
int main(){
deque<int> myDeque;
myDeque.push_back(90);
myDeque.push_back(80);
myDeque.push_back(70);
myDeque.push_back(60);
myDeque.push_back(50);
myDeque.push_back(40);
myDeque.push_back(30);
myDeque.push_back(20);
myDeque.push_back(10);
for (int i = 0; i < myDeque.size(); ++i){
if (i % 2 == 0){
cout << myDeque.at(i);
cout << " ";
}
}
return 0;
}

## Output

If we run the above code it will generate the following output −

90 70 50 30 10

## What is deque::swap()?

deque::swap() is an inbuilt function in C++ STL which is declared in <deque> header file. deque::swap() is used to swap the contents of one deque container with the other. This function takes an object of another deque container whose data we wish to swap with the associated deque container.

## Syntax

mydeque1.swap(type_t& mydeque2);

## Parameter

This function accepts one parameter i.e., the reference to the deque whose data we wish to swap with the associated deque.

## Return value

It returns nothing.

## Example

Input: deque<int> even = {2, 4, 6, 8};
deque<int> odd = {1, 3, 5, 7};
even.swap(odd);
Output:
Even deque: 1 3 5 7
Odd deque: 2 4 6 8

## Example

Live Demo

#include <deque>
#include <iostream>
using namespace std;
int main(){
deque<int> Deque_1 = { 1, 2, 3, 4 };
deque<int> Deque_2 = { 3, 5, 7, 9 };
Deque_1.swap(Deque_2);
cout<<"Deque_1 elements after swapping : ";
for (auto i = Deque_1.begin(); i< Deque_1.end(); ++i)
cout << *i << " ";
cout <<endl<<"Deque_2 elements after swapping : ";
for (auto i = Deque_2.begin(); i<Deque_2.end(); ++i)
cout << *i << " ";
return 0;
}

## Output

If we run the above code it will generate the following output −

Deque_1 elements after swapping : 3 5 7 9
Deque_2 elements after swapping : 1 2 3 4
Published on 05-Mar-2020 15:21:25