📅 Wed, Sep 18, 2024 ⏲️ ~ 7 minutes Advanced

What are Vectors in C++ and How to Use Them

In C++, a vector is a collection that implements a dynamically sized array (that is, it automatically adjusts in size as needed).

The std::vector class in the C++ standard library provides an implementation. Unlike traditional arrays, vectors allow for the addition and removal of elements.

If you want to learn more about Dynamic Arrays
check out the Introduction to Programming Course read more

Declaring Vectors

To declare a vector in C++, the following syntax is used:

#include <vector>

std::vector<type> vectorName;

Where:

type: This is the data type that the vector will contain, such as int, double, std::string, etc.
vectorName: This is the identifier for the vector.

For example, to declare a vector of integers:

#include <vector>

std::vector<int> numbers;

Creating and Initializing Vectors

Once a vector is declared, it must be initialized before use.

Empty Initialization

You can create an empty vector and then add elements to it:

#include <vector>

std::vector<int> numbers; // Empty vector

Initialization with Specific Size

You can initialize a vector with a specific size, where all elements are initialized to a default value (usually 0 for numeric types):

#include <vector>

std::vector<int> numbers(5); // Vector with 5 elements, all initialized to 0

Initialization with Specific Values

To initialize a vector with specific values, you can use the following syntax:

#include <vector>

std::vector<int> numbers = {1, 2, 3, 4, 5}; // Vector with values 1, 2, 3, 4, 5

Initialization with Value and Size

You can also initialize a vector with a specific size and an initial value for all elements:

#include <vector>

std::vector<int> numbers(5, 10); // Vector with 5 elements, all initialized to 10

Basic Usage of Vectors

Accessing Elements

Elements of a vector can be accessed using indices, starting from 0:

#include <vector>
#include <iostream>

int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5};
    int firstNumber = numbers[0]; // firstNumber will be 1
    std::cout << "The first number is: " << firstNumber << std::endl;

    return 0;
}

Modifying Elements

You can modify the elements of the vector by assigning new values to specific indices:

#include <vector>
#include <iostream>

int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5};
    numbers[1] = 20; // The second element of the vector will now be 20
    std::cout << "The modified second number is: " << numbers[1] << std::endl;

    return 0;
}

Adding Elements to a Vector

To add elements to the end of the vector, the push_back method is used:

#include <vector>
#include <iostream>

int main() {
    std::vector<int> numbers = {1, 2, 3};
    numbers.push_back(4); // Adds the number 4 to the end of the vector

    std::cout << "Vector after adding an element: ";
    for (int num : numbers) {
        std::cout << num << " ";
    }
    std::cout << std::endl;

    return 0;
}

Removing Elements from a Vector

To remove an element from the end of the vector, use the pop_back method:

#include <vector>
#include <iostream>

int main() {
    std::vector<int> numbers = {1, 2, 3, 4};
    numbers.pop_back(); // Removes the last element (4)

    std::cout << "Vector after removing an element: ";
    for (int num : numbers) {
        std::cout << num << " ";
    }
    std::cout << std::endl;

    return 0;
}

You can also remove an element at a specific position using the erase method:

#include <vector>
#include <iostream>

int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5};
    numbers.erase(numbers.begin() + 2); // Removes the third element (3)

    std::cout << "Vector after removing an element at position 2: ";
    for (int num : numbers) {
        std::cout << num << " ";
    }
    std::cout << std::endl;

    return 0;
}

Inserting Elements at a Specific Position

To insert an element at a specific position in the vector, use the insert method:

#include <vector>
#include <iostream>

int main() {
    std::vector<int> numbers = {1, 2, 4, 5};
    numbers.insert(numbers.begin() + 2, 3); // Inserts the number 3 at position 2

    std::cout << "Vector after inserting an element at position 2: ";
    for (int num : numbers) {
        std::cout << num << " ";
    }
    std::cout << std::endl;

    return 0;
}

Useful Properties and Methods

vectors in C++ offer various properties and methods for efficiently manipulating and accessing their elements:

Size Property

The size() function returns the number of elements in the vector:

#include <vector>
#include <iostream>

int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5};
    std::cout << "Number of elements in the vector: " << numbers.size() << std::endl;

    return 0;
}

Capacity Method

The capacity() function returns the current capacity of the vector, that is, the number of elements it can hold before needing a relocation:

#include <vector>
#include <iostream>

int main() {
    std::vector<int> numbers = {1, 2, 3};
    std::cout << "Capacity of the vector: " << numbers.capacity() << std::endl;

    return 0;
}

Clear Method

The clear() method removes all elements from the vector:

#include <vector>
#include <iostream>

int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5};
    numbers.clear(); // Removes all elements

    std::cout << "Number of elements after clear: " << numbers.size() << std::endl;

    return 0;
}

Sort Method

To sort the elements of a vector, you can use the sort function from the <algorithm> library:

#include <vector>
#include <algorithm>
#include <iostream>

int main() {
    std::vector<int> numbers = {5, 3, 1, 4, 2};
    std::sort(numbers.begin(), numbers.end()); // Sorts the vector in ascending order

    std::cout << "Sorted vector: ";
    for (int num : numbers) {
        std::cout << num << " ";
    }
    std::cout << std::endl;

    return 0;
}

Reverse Method

To reverse the order of the elements, you can use the reverse function from the <algorithm> library:

#include <vector>
#include <algorithm>
#include <iostream>

int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5};
    std::reverse(numbers.begin(), numbers.end()); // Reverses the order of elements

    std::cout << "Reversed vector: ";
    for (int num : numbers) {
        std::cout << num << " ";
    }
    std::cout << std::endl;

    return 0;
}

Practical Examples

Calculating the Sum of Elements in a Vector

This example uses a for loop to calculate the sum of the elements in a vector:

#include <vector>
#include <iostream>

int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5};
    int sum = 0;

    for (int num : numbers) {
        sum += num;
    }

    std::cout << "The sum of the elements is: " << sum << std::endl;

    return 0;
}

Filtering Elements from a Vector

This example uses a for loop to filter even elements from a vector:

#include <vector>
#include <iostream>

int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5};
    std::vector<int> evenNumbers;

    for (int num : numbers) {
        if (num % 2 == 0) {
            evenNumbers.push_back(num);
        }
    }

    std::cout << "Even numbers: ";
    for (int num : evenNumbers) {
        std::cout << num << " ";
    }
    std::cout << std::endl;

    return 0;
}

Finding the Maximum Value in a Vector

This example uses the max_element function from the <algorithm> library to find the maximum value in a vector:

#include <vector>
#include <algorithm>
#include <iostream>

int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5};
    int maximum = *std::max_element(numbers.begin(), numbers.end());

    std::cout << "The maximum value is: " << maximum << std::endl;

    return 0;
}

Counting How Many Elements Meet a Condition

This example uses the count_if function from the <algorithm> library to count how many elements in a vector are greater than a specific value:

#include <vector>
#include <algorithm>
#include <iostream>

int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5};
    int counter = std::count_if(numbers.begin(), numbers.end(), [](int num) { return num > 3; });

    std::cout << "There are " << counter << " elements greater than 3" << std::endl;

    return 0;
}

Removing Elements from a Vector

This example uses the remove_if function from the <algorithm> library to remove all odd elements from a vector:

#include <vector>
#include <algorithm>
#include <iostream>

int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5};
    numbers.erase(std::remove_if(numbers.begin(), numbers.end(), [](int num) { return num % 2 != 0; }), numbers.end());

    std::cout << "Remaining numbers: ";
    for (int num : numbers) {
        std::cout << num << " ";
    }
    std::cout << std::endl;

    return 0;
}