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Different ways to access elements from a Vector in C++ STL

Here, we are going to learn how to access elements from a vector by using different methods in C++ STL?
Submitted by Yash Khandelwal, on April 20, 2019

Different ways to access elements from a vector

1) Using [] operator

Syntax:

    vector_name[index]

Parameter: index – is the position in vector.(0-based indexing)

Return Value: The element at that given index.

Example:

    vector<int> a;

    a.push_back(1);
    a.push_back(2);
    a.push_back(3);

    int p=a[0]; //p=1
    int q=a[1]; //q=2

a[n] for any n out of index bound it reflects undefined behavior (depends on compiler).

2) Using function .at(index)

Syntax:

    vector_name.at(pos_n)

Parameter: index of desired element

Return value: Returns the referenced element present at input index.

It returns a reference to the element and this function automatically checks whether n is within the range or not. In case of exception it raises out of index range error.

That's the difference between .at() and [] operator does not keep any check for out of the index bounds.

Example:

    //On same vector a

    int p=a.at(0); //p=1
    int q=a.at(1); //q=2

    a.at(4) //compilation error

3) front() function

Syntax:

    vector_name.front()

Parameter: None

Return value: Return the reference of first element of the vector.

Example:

    //On same vector a

    int p=a.front() //p=1

Note: Calling this function on an empty vector shows undefined behavior.

4) back() function

Syntax:

    vector_name.back()

Parameter: None

Return: Reference of last element.

Example:

    //On same vector a

    int q=a.back() //q=3

Note: Calling this function on an empty vector shows undefined behavior.

C++ code to demonstrate example of the functions to access vector elements

// C++ program to show 
// how to access elements in vector in C++ STL>
#include <vector>
#include <iostream>
using namespace std;

int main( )
{
	// declaring vector n
	vector<int>n{1,2,3,4,5,6,7,8,9,0};
	
	/* This is how the operator[i]
	in c++ works i is the index
	which is changing continuously
	i.e printing the vector is best example*/

	cout <<"Output of operator[]: \n";
	for(int i=0; i<n.size(); i++)
	cout<<n[i]<<" ";
	
	/* This is how at() works similar
	to the operator[] ,
	Here it changes the whole vector
	*/
	cout << "\n\nOutput of at(): \n";
	for(int i=0; i<n.size(); i++)
	{
	n.at(i)=i;
	cout<<n.at(i)<<" ";
	}
	cout << "\n\nOutput of change vector because of at(): \n";
	for(int i=0; i<n.size(); i++)
	cout<<n[i]<<" ";

	
	/*This is how the front()
	works by using that here we are
	accessing the front element of vector
	*/
	cout << "\n\nOutput of front(): \n";
	cout<<n.front();


	
	/*This is how the back()
	works by using that here we are
	accessing the back element of vector
	*/
	cout << "\n\nOutput of back(): \n";
	cout<<n.back();

	return 0;
}

Output

Output of operator[]:
1 2 3 4 5 6 7 8 9 0

Output of at():
0 1 2 3 4 5 6 7 8 9

Output of change vector because of at():
0 1 2 3 4 5 6 7 8 9

Output of front():
0

Output of back():
9





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