vector::front() and vector::back() in C++ STL

Vectors are same as dynamic arrays with the ability to resize itself automatically when an element is inserted or deleted, with their storage being handled automatically by the container.

vector::front()

This function can be used to fetch the first element of a vector container.
Syntax :

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

Examples: 

Input  :  myvector = 1, 2, 3
          myvector.front();
Output :  1

Input  :  myvector = 3, 4, 1, 7, 3
          myvector.front();
Output :  3

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

Time Complexity – Constant O(1)

CPP

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

int main()
{
    vector<int> myvector;
    myvector.push_back(3);
    myvector.push_back(4);
    myvector.push_back(1);
    myvector.push_back(7);
    myvector.push_back(3);
    // Vector becomes 3, 4, 1, 7, 3

    cout << myvector.front();
    return 0;
}

Output: 

3

vector::back()

This function can be used to fetch the last element of a vector container.
Syntax :

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

Examples: 

Input  :  myvector = 1, 2, 3
          myvector.back();
Output :  3

Input  :  myvector = 3, 4, 1, 7, 2
          myvector.back();
Output :  2

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

Time Complexity – Constant O(1)

CPP

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

int main()
{
    vector<int> myvector;
    myvector.push_back(3);
    myvector.push_back(4);
    myvector.push_back(1);
    myvector.push_back(7);
    myvector.push_back(2);
    // Vector becomes 3, 4, 1, 7, 2

    cout << myvector.back();
    return 0;
}

Output: 

2

Difference between front(), back() and begin, end() function

begin() and end() function return an iterator(like a pointer) initialized to the first or the last element of the container that can be used to iterate through the collection, while front() and back() function just return a reference to the first or the last element of the container.
Application : Given an empty vector of integers, add numbers to the vector, 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 vector using 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.

CPP

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

int main()
{
    vector<int> myvector;
    myvector.push_back(8);
    myvector.push_back(7);
    myvector.push_back(6);
    myvector.push_back(5);
    myvector.push_back(4);
    myvector.push_back(3);
    myvector.push_back(2);
    myvector.push_back(1);

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

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

Output: 

7

Let us see the differences in a tabular form -:

	vector::front()	vector::back()
1.	It is used to return a reference to the first element in the vector.	It is used to return a reference to the last element in the vector.
2.	Its syntax is -: vectorName.front();	Its syntax is -: vectorName.back();
3.	It works on front (left) side of the vector.	It works on rear(right) side of the vector.
4.	It does not take any parameters.	It does not take any parameters.
5.	Its complexity is constant.	Its complexity is constant.
6.	Its iterator validity does not changes.	Its iterator validity does not changes.