list::front() and list::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::front()

This function is used to reference the first element of the list container. This function can be used to fetch the first element of a list.

Syntax :

listname.front()
Parameters :
No value is needed to pass as the parameter.
Returns :
Direct reference to the first element of the list container.

Examples: 

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

Input  : list list{0, 1, 2, 3, 4, 5};
         list.front();
Output : 0

Errors and Exceptions

1. If the list container is empty, it causes undefined behavior
2. It has a no exception throw guarantee if the list is not empty

C++

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

int main()
{
    list<int> mylist{ 1, 2, 3, 4, 5 };
    cout << mylist.front();
    return 0;
}

Output: 

1

list::back()

This function is used to reference the last element of the list container. This function can be used to fetch the first element from the end of a list.

Syntax :

listname.back()
Parameters :
No value is needed to pass as the parameter.
Returns :
Direct reference to the last element of the list container.

Examples:  

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

Input  : list list{1, 2, 3, 4, 5, 6};
         list.back();
Output : 6

Errors and Exceptions

1. If the list container is empty, it causes undefined behavior
2. It has a no exception throw guarantee if the list is not empty

C++

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

int main()
{
    list<int> mylist{ 1, 2, 3, 4, 5 };
    cout << mylist.back();
    return 0;
}

Output: 

5

Application
Given an empty list of integers, add numbers to the list, then print the difference between the first and the last element.

Input: 1, 2, 3, 4, 5, 6, 7, 8
Output:7
Explanation: Last element = 8, First element = 1, Difference = 7

Algorithm
1. Add numbers to the list using push_front() or push_back() function
2. Compare the first and the last element. 
3. If first element is larger, subtract last element from it and print it. 
4. Else subtract first element from the last element and print it.

C++

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

int main()
{
    list<int> mylist{};
    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

    if (mylist.front() > mylist.back()) {
        cout << mylist.front() - mylist.back();
    }
    else if (mylist.front() < mylist.back()) {
        cout << mylist.back() - mylist.front();
    }
    else
        cout << "0";
}

Output: 

7

Let us see the differences in a tabular form -:

	list::front()	list::back()
1.	It is used to return a reference to the first element in the list container	It is used to return a reference to the last element in the list container.
2.	Its syntax is -:  reference front();	Its syntax is -:  reference back();
3.	It does not take any parameters.	It does not take any parameters.
4.	Its complexity is constant.	Its complexity is constant.
5.	Its iterator validity does not changes.	Its iterator validity does not changes.