list::pop_front() and list::pop_back() in C++ STL

Lists are containers used in C++ to store data in a non contiguous fashion, Normally, Arrays and Vectors are contiguous in nature, therefore the insertion and deletion operations are costlier as compared to the insertion and deletion option in Lists.

list::pop_front()

pop_front() function is used to pop or remove elements from a list from the front. The value is removed from the list from the beginning, and the container size is decreased by 1. Syntax :

listname.pop_front()
Parameters :
No argument is passed as parameter.
Result :
Removes the value present at the front 
of the given list named as listname

Examples:

Input :  list list{1, 2, 3, 4, 5};
         list.pop_front();
Output : 2, 3, 4, 5

Input :  list list{5, 4, 3, 2, 1};
         list.pop_front();
Output : 4, 3, 2, 1

Errors and Exceptions

1. No-Throw-Guarantee – if an exception is thrown, there are no changes in the container.
2. If the list is empty, it shows undefined behaviour.

CPP

// CPP program to illustrate
// pop_front() function
#include <iostream>
#include <list>
using namespace std;

int main()
{
    list<int> mylist{ 1, 2, 3, 4, 5 };
    mylist.pop_front();

    // list becomes 2, 3, 4, 5

    for (auto it = mylist.begin(); it != mylist.end(); ++it)
        cout << ' ' << *it;
}

Output:

2, 3, 4, 5

Application : Input an empty list with the following numbers and order using push_front() function and print the reverse of the list.

Input : 1, 2, 3, 4, 5, 6, 7, 8
Output: 8, 7, 6, 5, 4, 3, 2, 1

CPP

// CPP program to illustrate
// application Of pop_front() function
#include <iostream>
#include <list>
using namespace std;

int main()
{
    list<int> mylist{}, newlist{};
    mylist.push_front(8);
    mylist.push_front(7);
    mylist.push_front(6);
    mylist.push_front(5);
    mylist.push_front(4);
    mylist.push_front(3);
    mylist.push_front(2);
    mylist.push_front(1);

    // list becomes 1, 2, 3, 4, 5, 6, 7, 8

    while (!mylist.empty()) {
        newlist.push_front(mylist.front());
        mylist.pop_front();
    }
    for (auto it = newlist.begin(); it != newlist.end(); ++it)
        cout << ' ' << *it;
}

Output:

8, 7, 6, 5, 4, 3, 2, 1

list::pop_back()

pop_back() function is used to pop or remove elements from a list from the back. The value is removed from the list from the end, and the container size is decreased by 1. Syntax :

listname.pop_back()
Parameters :
No argument is passed as parameter.
Result :
Removes the value present at the end or back 
of the given list named as listname

Examples:

Input :  list list{1, 2, 3, 4, 5};
         list.pop_back();
Output : 1, 2, 3, 4

Input :  list list{5, 4, 3, 2, 1};
         list.pop_back();
Output : 5, 4, 3, 2

Errors and Exceptions

1. No-Throw-Guarantee – if an exception is thrown, there are no changes in the container.
2. If the list is empty, it shows undefined behaviour.

CPP

// CPP program to illustrate
// pop_back() function
#include <iostream>
#include <list>
using namespace std;

int main()
{
    list<int> mylist{ 1, 2, 3, 4, 5 };
    mylist.pop_back();

    // list becomes 1, 2, 3, 4

    for (auto it = mylist.begin(); it != mylist.end(); ++it)
        cout << ' ' << *it;
}

Output:

1, 2, 3, 4

Application : Input an empty list with the following numbers and order using push_front() function and print the reverse of the list.

Input : 1, 20, 39, 43, 57, 64, 73, 82
Output: 82, 73, 64, 57, 43, 39, 20, 1

CPP

// CPP program to illustrate
// application Of pop_back() function
#include <iostream>
#include <list>
using namespace std;

int main()
{
    list<int> mylist{}, newlist{};
    mylist.push_front(82);
    mylist.push_front(73);
    mylist.push_front(64);
    mylist.push_front(57);
    mylist.push_front(43);
    mylist.push_front(39);
    mylist.push_front(20);
    mylist.push_front(1);

    // list becomes 1, 20, 39, 43, 57, 64, 73, 82

    while (!mylist.empty()) {
        newlist.push_back(mylist.back());
        mylist.pop_back();
    }
    for (auto it = newlist.begin(); it != newlist.end(); ++it)
        cout << ' ' << *it;
}

Output:

82, 73, 64, 57, 43, 39, 20, 1

Let us see the differences in a tabular form -:

	list::pop_front()	list::pop_back()
1.	It is used to remove the first element in the list container	It is used to remove the last element in the list container
2.	Its syntax is -:  pop_front();	Its syntax is -: pop_back();
3.	It does not take any parameters.	It does not take any parameters.
4.	It does not have any return type.	It does not have any return type.
5.	Its complexity is constant.	Its complexity is constant.