Vector in C++ STL

 Vector 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 elements are placed in contiguous storage so that they can be accessed and traversed using iterators. In vectors, data is inserted at the end. Inserting at the end takes differential time, as sometimes there may be a need of extending the array.Removing the last element takes only constant time, because no resizing happens. Inserting and erasing at the beginning or in the middle is linear in time.

Certain functions are associated with vector :
Iterators
1. begin() – Returns an iterator pointing to the first element in the vector
2. end() – Returns an iterator pointing to the theoretical element that follows last element in the vector
3. rbegin() – Returns a reverse iterator pointing to the last element in the vector (reverse beginning). It moves from last to first element
4. rend() – Returns a reverse iterator pointing to the theoretical element preceding the first element in the vector (considered as reverse end)

#include <iostream>

#include <vector>

using namespace std;

int main()

{

    vector <int> g1;

    vector <int> :: iterator i;

    vector <int> :: reverse_iterator ir;

    for (int i = 1; i <= 5; i++)

        g1.push_back(i);

    cout << "Output of begin and end\t:\t";

    for (i = g1.begin(); i != g1.end(); ++i)

        cout << *i << '\t';

    cout << endl << endl;

    cout << "Output of rbegin and rend\t:\t";

    for (ir = g1.rbegin(); ir != g1.rend(); ++ir)

        cout << '\t' << *ir;

    return 0;

}

Output of begin and end : 1 2 3 4 5

Output of rbegin and rend  : 5 4 3 2 1







Capacity

1. size() – Returns the number of elements in the vector

2. max_size() – Returns the maximum number of elements that the vector can hold

3. capacity() – Returns the size of the storage space currently allocated to the vector expressed as number of elements

4. resize(size_type g) – Resizes the container so that it contains ‘g’ elements

5. empty() – Returns whether the container is empty

#include <iostream>

#include <vector>

using namespace std;

int main()

{

    vector <int> g1;

    for (int i = 1; i <= 5; i++)

        g1.push_back(i);

    cout << "Size : " << g1.size();

    cout << "\nCapacity : " << g1.capacity();

    cout << "\nMax_Size : " << g1.max_size();

    return 0;

}






The output of the above program is :

Size : 5
Capacity : 8
Max_Size : 4611686018427387903







Accessing the elements

1. reference operator [g] – Returns a reference to the element at position ‘g’ in the vector

2. at(g) – Returns a reference to the element at position ‘g’ in the vector

3. front() – Returns a reference to the first element in the vector

4. back() – Returns a reference to the last element in the vector



#include <iostream>

#include <vector>

#include <string>

using namespace std;

int main()

{

    vector <int> g1;

    for (int i = 1; i <= 10; i++)

        g1.push_back(i * 10);

    cout << "Reference operator [g] : g1[2] = " << g1[2];

    cout << endl;

    cout << "at : g1.at(4) = " << g1.at(4);

    cout << endl;

    cout << "front() : g1.front() = " << g1.front();

    cout << endl;

    cout << "back() : g1.back() = " << g1.back();

    cout << endl;

    return 0;

}

The output of the above program is :

Reference operator [g] : g1[2] = 30 at : g1.at(4) = 50 front() : g1.front() = 10 back() : g1.back() = 100