C++ - 2 - Vectors

C++ vectors are dynamic arrays that can grow or shrink in size, making them a versatile data structure for storing elements of the same type. They are part of the C++ Standard Template Library (STL) and provide various functionalities that simplify memory management compared to traditional arrays.

Key Features of Vectors

Dynamic Sizing: Unlike arrays, the size of a vector can change during program execution. This allows for more flexible data handling.
Type Safety: Vectors can store elements of any data type, specified at the time of declaration.
Memory Management: Vectors automatically manage memory allocation and deallocation, reducing the risk of memory leaks.

Basic Syntax

To use vectors in C++, you must include the <vector> and the <algorithm> header file:

#include <vector>

#include <algorithm>

Declaration

Vectors are declared using the following syntax:

std::vector<T> vector_name;

Here, T is the data type of the vector elements. For example:

std::vector<int> numbers; // A vector of integers

Initialization

Vectors can be initialized in several ways:

Initializer List:

std::vector<int> vec1 = {1, 2, 3, 4, 5};

Uniform Initialization:

std::vector<int> vec2{1, 2, 3, 4, 5};

Specify Size and Default Value:

std::vector<int> vec3(5, 12); // A vector of size 5 initialized with value 12

To add elements to a vector in C++, you can use several methods provided by the Standard Template Library (STL). Here are the most common ways:

Methods to Add Elements

Using push_back():
- This method adds an element to the end of the vector.
- Syntax:

vector_name.push_back(value);

- Example:

std::vector<int> numbers;

numbers.push_back(10); // Adds 10 to the end of the vector

numbers.push_back(20); // Adds 20 to the end of the vector

Using emplace_back():
- Similar to push_back(), but it constructs the element in place, which can be more efficient for complex types.
- Syntax:

vector_name.emplace_back(value);

- Example:

std::vector<std::string> fruits;

fruits.emplace_back("Apple"); // Constructs "Apple" directly in the vector

Using insert():
- This method allows you to insert an element at a specified position within the vector.
- Syntax:

vector_name.insert(position, value);

- Example:

std::vector<int> numbers = {1, 2, 3};

numbers.insert(numbers.begin() + 1, 10); // Inserts 10 at index 1

Accessing Elements

Elements in a vector can be accessed using indices, similar to arrays. The recommended way is to use the .at() method, which provides bounds checking:

std::cout << vec1.at(0); // Accesses the first element

Alternatively, you can use the square bracket notation:

std::cout << vec1[0]; // Also accesses the first element but without bounds checking

Iterators

Vectors support iterators that allow traversal through elements. You can declare an iterator as follows:

std::vector<int>::iterator it;

To remove elements from a vector in C++, you can utilize several methods provided by the Standard Template Library (STL). Here’s a detailed overview of the most common techniques:

Methods to Remove Elements

Using pop_back():
- This method removes the last element from the vector.
- Syntax:

vector_name.pop_back();

- Example:

std::vector<int> numbers = {1, 2, 3};

numbers.pop_back(); // Removes the last element (3)

Using erase():
- This method can remove a single element or a range of elements specified by iterators.
- Syntax:
  - For a single element:

vector_name.erase(iterator position);

- - For a range:

vector_name.erase(iterator start, iterator end);

- Example:

std::vector<int> numbers = {1, 2, 3, 4};

numbers.erase(numbers.begin() + 1); // Removes the element at index 1 (2)

numbers.erase(numbers.begin(), numbers.end()); // Removes all elements

Using clear():
- This method removes all elements from the vector, leaving it empty.
- Syntax:

vector_name.clear();

- Example:

std::vector<int> numbers = {1, 2, 3};

numbers.clear(); // Removes all elements

Using remove():
- This algorithm removes elements that match a specified value but does not change the size of the vector. It shifts elements to fill the gaps and returns an iterator to the new end.
- Syntax:

std::remove(vector_name.begin(), vector_name.end(), value);

- After using remove(), you should call erase() to actually remove the elements from the vector.
- Example:

std::vector<int> numbers = {1, 2, 3, 2, 4};

auto new_end = std::remove(numbers.begin(), numbers.end(), 2);

numbers.erase(new_end, numbers.end()); // Removes all occurrences of 2

Using remove_if():
- This function removes elements for which a specified condition is true. Similar to remove(), it requires a subsequent call to erase().
- Syntax:

std::remove_if(vector_name.begin(), vector_name.end(), predicate);

- Example:

std::vector<int> numbers = {1, 2, 3, 4};

auto new_end = std::remove_if(numbers.begin(), numbers.end(), [](int x) { return x % 2 == 0; });

numbers.erase(new_end, numbers.end()); // Removes all even numbers

Example:

#include <iostream>

#include <vector>

#include <algorithm>

using namespace std;

int main() {

// 1. Initializing a vector

vector<int> numbers = {1, 2, 3, 4, 5}; // Using initializer list

// 2. Adding elements

numbers.push_back(6); // Add element at the end

numbers.push_back(7); // Add another element

// 3. Accessing elements

cout << "Accessing elements:" << endl;

for (size_t i = 0; i < numbers.size(); ++i) {

cout << "Element at index " << i << ": " << numbers[i] << endl; // Access using index

}

// 4.A Iterating over elements using range based for loop

cout << "Iterating over elements:" << endl;

for (int value : numbers) {

cout << value << " "; // Range-based for loop

}

cout << endl;

// 4.B Iterating over elements using normal for loop

for (vector<int>::iterator it = numbers.begin(); it != numbers.end(); it++) {

cout << *it << " "; // Dereferencing iterator to access value

}

cout << endl;

// 5. Removing elements

numbers.pop_back(); // Remove the last element (7)

// Remove specific element (e.g., remove all occurrences of 2)

auto new_end = remove(numbers.begin(), numbers.end(), 2);

numbers.erase(new_end, numbers.end()); // Actually removes the elements

// Displaying final state of the vector

cout << "Final vector after removals:" << endl;

for (int value : numbers) {

std::cout << value << " "; // Final state of the vector

}

return 0;

}

Exercise:

Sort an array of elements using a vector. The vector conatins the following elements:

vector<int> numbers = {7,5,22,12,15,3,5};

Using structures in vectors in C++ allows you to create collections of complex data types. Below is an example that demonstrates how to define a structure, initialize a vector of that structure, add elements, access elements, iterate over them, and remove elements.

Example Code

#include <iostream>

#include <vector>

#include <algorithm>

using namespace std;

// Define a structure

struct Person {

string name;

int age;

// Constructor for easier initialization

Person(const string& n, int a) : name(n), age(a) {}

};

// Function to manipulate the vector of Person structs

void manipulateVector() {

// 1. Initializing a vector of Person

vector<Person> people = {

Person("Alice", 30),

Person("Bob", 25),

Person("Charlie", 35)

};

// 2. Adding elements

people.push_back(Person("David", 28)); // Add a new person

// 3. Accessing elements

cout << "Accessing elements:" << endl;

for (size_t i = 0; i < people.size(); ++i) {

cout << "Person at index " << i << ": "

<< people[i].name << ", Age: " << people[i].age << endl;

}

// 4. Iterating over elements

cout << "\nIterating over elements:" << endl;

for (const auto& person : people) {

cout << person.name << " is " << person.age << " years old." << endl;

}

// 5. Removing elements

// Remove the first person (Alice)

people.erase(people.begin());

// Remove all persons aged 25

auto new_end = remove_if(people.begin(), people.end(), [](const Person& p) {

return p.age == 25;

});

people.erase(new_end, people.end()); // Actually removes the elements

// Display final state of the vector

cout << "\nFinal vector after removals:" << endl;

for (const auto& person : people) {

cout << person.name << " is " << person.age << " years old." << endl;

}

int main() {

manipulateVector(); // Call the function to manipulate the vector

return 0;

}

Using vectors in conjunction with classes in C++ allows for the creation of dynamic collections of objects.

Example:

#include <iostream>

#include <vector>

#include <algorithm>

using namespace std;

// Define a class representing a Student

class Student {

public:

string name;

int age;

// Constructor

Student(const string& n, int a) : name(n), age(a) {}

// Method to display student information

void display() const {

cout << "Name: " << name << ", Age: " << age << endl;

}

};

// Function to manipulate the vector of Student objects

void manipulateStudents() {

// 1. Initializing a vector of Student

vector<Student> students = {

Student("Alice", 20),

Student("Bob", 22),

Student("Charlie", 21)

};

// 2. Adding elements

students.push_back(Student("David", 23)); // Add a new student

// 3. Accessing elements

cout << "Accessing students:" << endl;

for (size_t i = 0; i < students.size(); ++i) {

students[i].display(); // Access using index and display information

}

// 4. Iterating over elements

cout << "\nIterating over students:" << endl;

for (const auto& student : students) {

student.display(); // Range-based for loop to display each student's info

}

// 5. Removing elements

// Remove the first student (Alice)

students.erase(students.begin());

// Remove all students aged 22 (Bob)

auto new_end = remove_if(students.begin(), students.end(), [](const Student& s) {

return s.age == 22; // Condition to remove Bob

});

students.erase(new_end, students.end()); // Actually removes the elements

// Display final state of the vector

cout << "\nFinal list of students after removals:" << endl;

for (const auto& student : students) {

student.display(); // Display remaining students' info

}

int main() {

manipulateStudents(); // Call the function to manipulate the vector of Student objects

return 0;

}

Example: Reading a CSV File using vectors and structures

data.csv

name,phone,email

test,123456789,[email protected]

test,213456789,[email protected]

#include <iostream>

#include <string>

#include <fstream>

#include <sstream>

#include <vector>

#include <algorithm>

using namespace std;

struct Person{

string name,phone,email;

};

int main()

{

vector<Person> pdata;

int count=1;

fstream f;

f.open("data.csv", ios::in);

string line, col;

bool first_line= true;

while (getline(f,line))

{

if (first_line)

{

first_line=false;

continue;

}

stringstream s(line);

Person p;

while (getline(s,col,','))

{

if(count==1){

p.name=col; count++;

}else if(count == 2){

p.phone=col; count++;

}else{

p.email=col; count=1;

}

pdata.push_back(p);

}

for(int i=0;i<pdata.size();i++){

Person p=pdata.at(i);

cout << p.name <<" "<<p.phone<<" "<<p.email<<endl;

}

return 0;

}