deque::clear() and deque::erase() 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::clear()

clear() function is used to remove all the elements of the deque container, thus making its size 0.
Syntax :

dequename.clear()
Parameters :
No parameters are passed.
Result :
All the elements of the deque are
removed ( or destroyed )

Examples:

Input  : mydeque = {1, 2, 3, 4, 5}
         mydeque.clear();
Output : mydeque = {}

Input  : mydeque = {}
         mydeque.clear();
Output : mydeque = {}

Errors and Exceptions

1. It has a no exception throw guarantee.
2. Shows error when a parameter is passed.

// CPP program to illustrate
// Implementation of clear() function
#include <deque>
#include <iostream>
using namespace std;

int main()
{
    deque<int> mydeque{ 1, 2, 3, 4, 5 };

    mydeque.clear();
    // Deque becomes empty

    // Printing the deque
    for (auto it = mydeque.begin(); it != mydeque.end(); ++it)
        cout << ' ' << *it;
    return 0;
}

Output:

No Output
deque::erase()

erase() function is used to remove elements from a container from the specified position or range.

Syntax :

1. dequename.erase(position)
2. dequename.erase(startingposition, endingposition)
Parameters :
Position of the element to be removed in the form of iterator.
or the range specified using start and end iterator.
Result :
Elements are removed from the specified
position of the container.

Examples:

Input  : mydeque{1, 2, 3, 4, 5}, iterator= 2
         mydeque.erase(iterator);
Output : 1, 2, 4, 5

Input  : mydeque{1, 2, 3, 4, 5, 6, 7, 8}, iterator1= 3, iterator2= 6
         mydeque.erase(iterator1, iterator2);
Output : 1, 2, 3, 8

Errors and Exceptions

1. It has a no exception throw guarantee, if the position is valid.
2. Shows undefined behaviour otherwise.

Removing element from particular position

// CPP program to illustrate
// Implementation of erase() function
#include <deque>
#include <iostream>
using namespace std;

int main()
{
    deque<int> mydeque{ 1, 2, 3, 4, 5 };
    deque<int>::iterator it;

    it = mydeque.begin();
    mydeque.erase(it);

    // Printing the deque
    for (auto it = mydeque.begin(); it != mydeque.end(); ++it)
        cout << ' ' << *it;
    return 0;
}

Output:

2 3 4 5

Removing elements within a range

// CPP program to illustrate
// Implementation of erase() function
#include <deque>
#include <iostream>
using namespace std;

int main()
{
    deque<int> mydeque{ 1, 2, 3, 4, 5 };
    deque<int>::iterator it1, it2;

    it1 = mydeque.begin();
    it2 = mydeque.end();
    it2--;
    it2--;

    mydeque.erase(it1, it2);

    // Printing the deque
    for (auto it = mydeque.begin(); it != mydeque.end(); ++it)
        cout << ' ' << *it;
    return 0;
}

Output:

4 5

Application
Given a list of integers, remove all the even elements from the deque and print the deque.

Input  :1, 2, 3, 4, 5, 6, 7, 8, 9
Output :1 3 5 7 9
Explanation - 2, 4, 6 and 8 which are even are erased from the deque

Algorithm
1. Run a loop till the size of the deque.
2. Check if the element at each position is divisible by 2, if yes, remove the element and decrement iterator.
3. Print the final deque.

// CPP program to illustrate
// Application of erase() function
#include <deque>
#include <iostream>
using namespace std;

int main()
{
    deque<int> mydeque{ 1, 2, 3, 4, 5, 6, 7, 8, 9 };

    for (auto i = mydeque.begin(); i != mydeque.end(); ++i) {
        if (*i % 2 == 0) {
            mydeque.erase(i);
            i--;
        }
    }

    // Printing the deque
    for (auto it = mydeque.begin(); it != mydeque.end(); ++it)
        cout << ' ' << *it;
    return 0;
}

Output:

1 3 5 7 9
clear() VS erase() . When to use what?

clear() removes all the elements from a deque container, thus making its size 0. All the elements of the deque are removed using clear() function.
erase() function on the other hand, is used to remove specific elements from the container or a range of elements from the container, thus reducing its size by the number of elements removed.


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