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::emplace_front()
This function is used to insert a new element into the deque container, the new element is added to the beginning of the deque.
Syntax :
dequename.emplace_front(value)
Parameters :
The element to be inserted into the deque
is passed as the parameter.
Result :
The parameter is added to the
deque at the beginning.
Examples:
Input : mydeque{1, 2, 3, 4, 5};
mydeque.emplace_front(6);
Output : mydeque = 6, 1, 2, 3, 4, 5
Input : mydeque{};
mydeque.emplace_front(4);
Output : mydeque = 4
Errors and Exceptions
1. It has a strong exception guarantee, therefore, no changes are made if an exception is thrown.
2. The parameter should be of the same type as that of the container, otherwise, an error is thrown.
CPP
#include <deque>
#include <iostream>
using namespace std;
int main()
{
deque<int> mydeque;
mydeque.emplace_front(1);
mydeque.emplace_front(2);
mydeque.emplace_front(3);
mydeque.emplace_front(4);
mydeque.emplace_front(5);
mydeque.emplace_front(6);
for (auto it = mydeque.begin();
it != mydeque.end(); ++it)
cout << ' ' << *it;
return 0;
}
|
Output:
6 5 4 3 2 1
CPP
#include <deque>
#include <iostream>
#include <string>
using namespace std;
int main()
{
deque<string> mydeque;
mydeque.emplace_front("portal");
mydeque.emplace_front("science");
mydeque.emplace_front("computer");
mydeque.emplace_front("a");
mydeque.emplace_front("is");
mydeque.emplace_front("GEEKSFORGEEKS");
for (auto it = mydeque.begin();
it != mydeque.end(); ++it)
cout << ' ' << *it;
return 0;
}
|
Output:
GEEKSFORGEEKS is a computer science portal
Time Complexity : O(1)
Application
Given an empty deque, add integers to it using emplace_front() function and then calculate its size without using size function().
Input : 1, 2, 3, 4, 5, 6
Output : 6
Algorithm
1. Add elements to the deque using emplace_front() function
2. Check if the deque is empty, if not, increment the counter variable initialised as 0, and pop the back element.
3. Repeat this step until the deque becomes empty.
4. Print the final value of the variable.
CPP
#include <deque>
#include <iostream>
using namespace std;
int main()
{
int count = 0;
deque<int> mydeque;
mydeque.emplace_front(1);
mydeque.emplace_front(2);
mydeque.emplace_front(3);
mydeque.emplace_front(4);
mydeque.emplace_front(5);
mydeque.emplace_front(6);
while (!mydeque.empty()) {
count++;
mydeque.pop_back();
}
cout << count;
return 0;
}
|
Output :
6
deque::emplace_back()
This function is used to insert a new element into the deque container, the new element is added to the end of the deque.
Syntax :
dequename.emplace_back(value)
Parameters :
The element to be inserted into the deque
is passed as the parameter.
Result :
The parameter is added to the
deque at the end.
Examples:
Input : mydeque{1, 2, 3, 4, 5};
mydeque.emplace_back(6);
Output : mydeque = 1, 2, 3, 4, 5, 6
Input : mydeque{};
mydeque.emplace_back(4);
Output : mydeque = 4
Errors and Exceptions
1. It has a strong exception guarantee, therefore, no changes are made if an exception is thrown.
2. Parameter should be of same type as that of the container, otherwise an error is thrown.
CPP
#include <deque>
#include <iostream>
using namespace std;
int main()
{
deque<int> mydeque;
mydeque.emplace_back(1);
mydeque.emplace_back(2);
mydeque.emplace_back(3);
mydeque.emplace_back(4);
mydeque.emplace_back(5);
mydeque.emplace_back(6);
for (auto it = mydeque.begin();
it != mydeque.end(); ++it)
cout << ' ' << *it;
return 0;
}
|
Output:
1 2 3 4 5 6
CPP
#include <deque>
#include <iostream>
using namespace std;
int main()
{
deque<string> mydeque;
mydeque.emplace_back("Hi");
mydeque.emplace_back("this");
mydeque.emplace_back("is");
mydeque.emplace_back("geeksforgeeks");
for (auto it = mydeque.begin();
it != mydeque.end(); ++it)
cout << ' ' << *it;
return 0;
}
|
Output:
Hi this is geeksforgeeks
Time Complexity : O(1)
Application
Given an empty deque, add integers to it using emplace_front() function and then calculate the sum of the elements.
Input : 4, 5, 9, 2, 6
Output : 26
Algorithm
1. Add elements to the deque using emplace_back() function
2. Check if the deque is empty, if not, add the back value to the sum variable initialised as 0, and pop the back element.
3. Repeat this step until the deque becomes empty.
4. Print the final value of the variable.
CPP
#include <deque>
#include <iostream>
using namespace std;
int main()
{
int sum = 0;
deque<int> mydeque;
mydeque.emplace_back(4);
mydeque.emplace_back(5);
mydeque.emplace_back(9);
mydeque.emplace_back(2);
mydeque.emplace_back(6);
while (!mydeque.empty()) {
sum = sum + mydeque.back();
mydeque.pop_back();
}
cout << sum;
return 0;
}
|
Output :
6
Time complexity: O(n), // n is the number of elements in the dequeue.
Auxiliary Space: O(n).
Let us see the differences in a tabular form -:
| deque::emplace_front() | deque::emplace_back() |
| 1. | It is used to insert a new element at the beginning of the deque | It is used to insert a new element at the end of the deque |
| 2. | Its syntax is -: emplace_front (Args&&… args); | Its syntax is -: emplace_back (Args&&… args); |
| 3. | It does not have any return value. | It does not have any return value. |
| 4. | Its complexity is constant. | Its complexity is constant. |
| 5. | It is present in <deque> header file | It is present in <deque> header file |