Vectors in C++ STL

• In vector we insert as many elements we want to add but in a array  of 10 integer to add the 11th element we need to make an array again.
• It is likely to array but array has static data type and vector has dynamic datatype.
• Vector is declared in the header file (#include<vector>).
• We use vector in C++ because it is dynamic and it automatically adjust the size when an element is inserted or deleted from it.

C++ Vector Declaration

vector<object_type> variable_name; // vector declaration

//Examples
vector<int> number;
vector<string> name;
vector <float> marks;

C++ Vector Initialization

There are different ways to initialize a vector in C++.

//initializing the vector by providing values directly to the vector.
vector<int> vector1= {1, 2, 3, 4, 5};   // first way

//vector with size 5 and initializes the vector with the value of 10
vector<int> vector2(5, 10);  //second way which is equivalent to <int> vector3 = {10, 10, 10, 10, 10};

Example of C++ Vector Initialization

#include <iostream>
#include <vector>
using namespace std;

int main() {

  // method 1
  vector<int> vector1 = {1, 2, 3, 4, 5};

  // method 2
  vector<int> vector2(5, 10);

  cout << "vector1 = ";

 // ranged loop, which is mostly used to retrieve the elements of vectors
  for (int i : vector1) {
    cout << i << "  ";
  }

  cout << "\nvector2 = ";

  // ranged loop
  for (int i : vector2) {
    cout << i << "  ";
  }

  return 0;
}

Output

vector1 = 1  2  3  4  5  
vector2 = 10  10  10  10  10  

Most used functions in Vector

• push_back() – it accepts a parameter and insert the element passed in the parameter in the vectors, the element is inserted at the end.

vector<int> v1;
v1.push_back(1);
v1.push_back(2);

• pop_back() – it deletes the last element and returns it to the calling function.

v1.pop_back();

• begin() – it returns an iterator pointing to the first element of the vector.

auto iterator = itr;
itr = v1.begin();

• end() – it returns an iterator pointing to the element theoretically after the last element of the vector.

auto iterator = itr;
itr = v1.end();

• insert() – it is used to insert an element at a specified position.

auto it= v1.begin();
v1.insert(it,5); //inserting 5 at the beginning

• erase() – it is used to delete a specific element

vector<int> v1;
auto it= v1.begin();
v1.erase(it);// erasing the first element

• front() – it returns a reference to the first element of the vector.

v1.front();

• back() – it returns a reference to the last element of the vector.

v1.back();

• clear() – deletes all the elements from the vector.

v1.clear();

• empty() – to check if the vector is empty or not.

v1.empty();

• size() – returns the size of the vector

v1.size();

Example of Vectors in C++

#include <iostream>
#include <vector>
using namespace std;

int main() {
  vector < int > v;

  for (int i = 0; i < 10; i++) {
    v.push_back(i); //inserting elements in the vector
  }


  cout << "the elements in the vector: ";
  for (auto it = v.begin(); it != v.end(); it++)
    cout << * it << " ";   // the elements in the vector: 0 1 2 3 4 5 6 7 8 9 

  cout << "\nThe front element of the vector: " << v.front();  //the front element of the vector: 0
  cout << "\nThe last element of the vector: " << v.back();    // the last element of the vector: 9
  cout << "\nThe size of the vector: " << v.size();   // the size of the vector: 10
  cout << "\nDeleting element from the end: " << v[v.size() - 1];  //deleting element from the end: 9
  v.pop_back();

  cout << "\nPrinting the vector after removing the last element:" << endl;
  for (int i = 0; i < v.size(); i++)
    cout << v[i] << " ";

//printing the vector after removing the last element: 0 1 2 3 4 5 6 7 8 

  cout << "\nInserting 5 at the beginning:" << endl;
  v.insert(v.begin(), 5);
  cout << "The first element is: " << v[0] << endl;  //the first element is: 5
  cout << "Erasing the first element" << endl;
  v.erase(v.begin());
  cout << "Now the first element is: " << v[0] << endl;  //now the first element is: 0

  if (v.empty())
    cout << "\nvector is empty";
  else
    cout << "\nvector is not empty" << endl;

  v.clear();
  cout << "Size of the vector after clearing the vector: " << v.size();  //size of the vector after clearing the vector: 0

}

Output:

the elements in the vector: 0 1 2 3 4 5 6 7 8 9
The front element of the vector: 0
The last element of the vector: 9
The size of the vector: 10
Deleting element from the end: 9
Printing the vector after removing the last element:
0 1 2 3 4 5 6 7 8
Inserting 5 at the beginning:
The first element is: 5
Erasing the first element
Now the first element is: 0
vector is not empty
Size of the vector after clearing the vector: 0

Others Function used as Vectors in C++

• assign() – It assigns new value to the vector elements by replacing old ones
• swap() – It is used to swap the contents of one vector with another vector of same type. Sizes may differ.
• clear() – It is used to remove all the elements of the vector container
• emplace() – It extends the container by inserting new element at position
• emplace_back() – It is used to insert a new element into the vector container, the new element is added to the end of the vector
• rbegin() – Returns a reverse iterator pointing to the last element in the vector (reverse beginning). It moves from last to first element
• rend() – Returns a reverse iterator pointing to the theoretical element preceding the first element in the vector (considered as reverse end)
• cbegin() – Returns a constant iterator pointing to the first element in the vector.
• cend() – Returns a constant iterator pointing to the theoretical element that follows the last element in the vector.
• crbegin() – Returns a constant reverse iterator pointing to the last element in the vector (reverse beginning). It moves from last to first element
• crend() – Returns a constant reverse iterator pointing to the theoretical element preceding the first element in the vector (considered as reverse end)

Time complexity for various operations on vectors

• Random access – constant O(1)
• Insertion or removal of elements at the end – constant O(1)
• Insertion or removal of elements – linear in the distance to the end of the vector O(N)
• Knowing the size – constant O(1)
• Resizing the vector- Linear O(N)