Vector in C++

Introduction

• Vectors are part of STL.
• Vectors are sequence containers representing arrays that can change in size.
• Vectors use contiguous storage locations for their elements, so their elements can also be accessed using offsets on regular pointers to their elements.
• Vector size can change dynamically, with their storage being handled automatically by the container.
• Vectors use a dynamically allocated array to store their elements.

Internal Storage

• Vectors use a dynamically allocated array to store their elements.
• This array may need to be reallocated in order to grow in size when new elements are inserted (allocate a new array and move all elements to it) and it is expensive in terms of processing time.
• Vector containers may allocate some extra storage to accommodate for possible growth.
• Compared to arrays, vectors consume more memory in exchange for the ability to manage storage and grow dynamically.

When to Use Vectors?

• Data is consistently changing.
• The size of data is unknown.
• Elements are not predefined.
• Compared to arrays, there are more ways to copy vectors.

Declaration

Include #include<vector> library.

// Syntax: vector<object_type> vector_variable_name;
#include <vector>
using namespace std;
vector<int> numbers;

Initialization

1. Pushing the values one-by-one in vector using push_back():

// Syntax: vector_name.push_back(element_value);
numbers.push_back(3);
numbers.push_back(8);
// numbers = { 3, 8 } 

2. Using the overload constructor of the vector Class:

// Syntax: vector<object_type> vector_name(number_of_repetition, element_value);
vector<int> number1(2, 3);
// numbers1 = { 3, 3 }

3. Using Array:

// Syntax: vector<object_type> vector_name {val1,val2,....,valn};
vector<int> number2 { 9, 3, 6 };
// numbers2 = { 9, 3, 6 }

4. Using already initialized vector:

// Syntax: vector<object_type> vector_name_2(vector_name_1.begin(),vector_name_1.end());
vector<int> number3(number2.begin(), number2.end());
// numbers3 = { 9, 3, 6 }

Member Functions

Iterators

• begin(): Returns an iterator pointing to the first element in the vector.
• end(): Returns an iterator pointing to the last element in the vector.

vector<int> a = { 3, 4, 5 };
for (auto i = a.begin(); i != a.end(); ++i) {
    cout << *i << " ";
}
cout << endl;

Output:

3 4 5

Capacity

• size(): Returns the number of elements currently present in the vector.
• max_size(): Returns the maximum number of elements that a vector can hold.
• capacity(): Returns the storage capacity currently allocated to the vector.
• resize(n): Resizes the container to store n elements.
• empty(): Returns whether the container is empty or not.

vector<int> b = { 5, 7, 1 };
cout << b.size() << endl;
cout << b.max_size() << endl;
cout << b.capacity() << endl;
b.resize(5);
cout << b.size() << endl;
cout << b.empty() << endl;

Output:

3
2305843009213693951
3
5
0