C++ STL: vector (Complete Guide)

Vectors are sequential containers that are very similar to arrays. They are continuous blocks of memory. Unlike arrays, vectors can have dynamic sizes. Vectors are one of the most useful containers in STL. Vector is present in #include<vector>header file. The elements inside the vector can be accessed using iterators. 

Vector Declaration

Syntax:

vector<data_type> name(size,initial_value);

size: optional parameter which tells us the size of the vector

initial_value: optional parameter which initializes the vector with the given value

Example:

vector < int > v; //initializes a vector of size 0 which stores integer values 
vector < int > v(10); //initializes a vector of size 10 having initial value as 0
vector < int > v(10, 1); //initializes a vector of size 10 having initial value as 1

Functions on vectors

• push_back(element): It pushes the value at the end of the vector. 
  Time Complexity: O(1) 
  Parameters: the element which is to be inserted
  Return type: void
   
• size(): It tells us the size of the vector.
  Time Complexity: O(1)
  Parameters: None
  Return type: integer - total number of elements in the vector
   
• pop_back(): It deletes the last element of the vector. 
  Time Complexity: O(1)
  Parameters: None
  Return type: void
   
• clear(): It deletes all the elements from the vector.
  Time Complexity: O(N) where N is the size of the vector
  Parameters: None
  Return type: void
   
• empty(): It tells us whether the vector is empty or not.
  Time Complexity: O(1)
  Parameters: None
  Return type: Boolean, true if vector is empty else false

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

void print(vector < int > v) {
  for (int i = 0; i < v.size(); i++) {
    cout << v[i] << " ";
  }
  cout << '\n';
  return;
}
int main() {
  vector < int > v;
  for (int i = 0; i < 5; i++) {
    v.push_back(i + 1);
  }
  cout << "Size of vector is " << v.size() << '\n';
  print(v);
  v.pop_back();
  print(v);
  v.clear();
  if (v.empty() == true) {
    cout << "vector is empty!";
  }
  return 0;
}

Output

Size of vector is 5
1 2 3 4 5
1 2 3 4
vector is empty!

Copying one vector to another

vector < int > v2 = v1; //copies vector1 into vector2	

Any change in v2 will not affect v1. The time complexity of this operation is O(N) where N is the size of the vector1. The same can be done using reference but it will not create a copy, it will contain a reference to the original vector.

vector < int > & v2 = v1; //v2 references v1

Passing vector by value to a function

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

void print(vector < int > v) {
  for (int i = 0; i < v.size(); i++) {
    cout << v[i] << " ";
  }
  return;
}
int main() {
  int n;
  cin >> n;
  vector < int > v(n);
  for (int i = 0; i < n; i++) {
    int x;
    cin >> x;
    v.push_back(x);
  }
  print(v);
  return 0;
}

When the vector v is passed to the print function from the main function, a copy is created which takes O(N) time.

Passing vector by reference to a function

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

void print(vector < int > & v) {
  for (int i = 0; i < v.size(); i++) {
    cout << v[i] << " ";
  }
  return;
}
int main() {
  int n;
  cin >> n;
  vector < int > v(n);
  for (int i = 0; i < n; i++) {
    int x;
    cin >> x;
    v.push_back(x);
  }
  print(v);
  return 0;
}

When the vector v is passed to the print function from the main function, the reference to vector v is passed. This prevents us from creating a copy of the vector.

Nesting in vectors

Array of vectors

vector < int > arr[10]; //an array arr whose each element is a vector

An array of vectors behaves like a 2D array but it has variable size columns.

vector of vectors

vector < vector < int >> v; //a vector v whose each element is a vector

vector of vectors behaves like a 2D array but it has variable size rows and columns.