# Eigen C++ Tutorial – Complete Guide

Welcome to an exciting journey into the world of Eigen C++, a high-level C++ library for linear algebra! This tutorial is your chance to get hands-on with Eigen, understand how it works, and discover the power it lends to your programming toolkit.

## What is Eigen?

Eigen is a powerful C++ library designed to simplify the complexities of linear algebra computations for programmers. It introduces the precision and speed of linear algebra computations to a broad range of possibilities for programming tasks, artificial intelligence, and game development offer.

## Why should you learn Eigen C++?

Matrix manipulations, vector arithmetic, numerical solvers, and related algorithms are pervasive in computational tasks. Learning Eigen C++ empowers you to perform these complex calculations with relative ease, enhancing the versatility of your code especially in game physics and AI.

## What can you do with Eigen?

From solving systems of linear equations to implementing transformations in 3D games, Eigen packs a punch when it comes to handling mathematical heavy-lifting. This positions it as a versatile tool not just for game developers but also for those delving into the realm of machine learning and AI. Now that you have a basic understanding of what Eigen C++ is and why it’s worth learning, let’s dive into some hands-on coding!

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## Setting up Eigen

First, we’ll need to download and install the Eigen library. Once we have the library installed, we can include Eigen in our C++ programs like any other header file. Assuming you have the Eigen library installed, here’s how you can include it:

`#include <Eigen/Dense>`

## Working with Matrices

In Eigen, a matrix is a rectangular array of numbers. Here’s an example of how you can declare a matrix:

`Eigen::Matrix3f m3;`

This code creates a 3×3 matrix of floating-point numbers. We can also initialize a matrix at declaration like this:

```Eigen::Matrix3f m3;
m3 << 1, 2, 3,
4, 5, 6,
7, 8, 9;```

The above code initializes 3×3 matrix with values ranging from 1 to 9.

## Performing Matrix Operations

With Eigen, you can do all standard matrix operations. Let’s take a look at matrix addition:

```Eigen::Matrix2f a, b;
a << 1, 2,
3, 4;
b << 5, 6,
7, 8;
std::cout << "a + b =\n" << a + b << std::endl;```

This code creates two 2×2 matrices, adds them together, and prints the result, which is another 2×2 matrix.

## Working with Vectors

In Eigen, vectors are just special case of matrices. Let’s declare a 3-dimensional vector:

`Eigen::Vector3f v;`

The above code declares a 3-dimensional vector of floating-point numbers. You can also initialize a vector at declaration, similar to matrices:

`Eigen::Vector3f v(1,2,3);`

This will declare and initialize a vector with the given values.

## Performing Vector Operations

Just like with matrices, you can also perform standard vector operations using Eigen. Here’s an example of vector addition:

```Eigen::Vector3f v1(1,2,3);
Eigen::Vector3f v2(4,5,6);
std::cout << "v1 + v2 =\n" << v1 + v2 << std::endl;```

This creates two 3D vectors, adds them together, and prints the result, which is another 3D vector.

How about scalar multiplication? Here’s an example:

```Eigen::Vector3f v(1,2,3);
std::cout << "v * 3 =\n" << v * 3 << std::endl;```

This creates a 3D vector, multiplies it by a scalar (3 in this case), and prints the result.

## Eigenvalues and Eigenvectors

Eigen can also compute eigenvalues and eigenvectors. These are important concepts in many areas, including machine learning and computer graphics. Let’s declare a 3×3 matrix and compute its eigenvalues:

```Eigen::Matrix3f m;
m << 1, 2, 1,
2, 1, 0,
-1, 1, 2;
Eigen::EigenSolver solver(m);
std::cout << "The eigenvalues of m are:\n" << solver.eigenvalues() << std::endl;```

This will print the eigenvalues of the declared matrix. Typically this process is used in advanced computations.

## Conclusions

As we have explored, Eigen is a powerful tool for performing complex linear algebra operations in C++. If you are working in machine learning, computer graphics, game development, or any other field that involves heavy number crunching, learning Eigen can be a great asset.

Zenva strongly believes in the power of continuous learning and providing high-quality content. Therefore, we encourage you to delve deeper into Eigen, practice with the library, and discover the numerous possibilities it can offer you in your coding journey.

## Matrix and Vector Multiplication

Perhaps one of the most common operations in linear algebra is the multiplication of matrices and vectors. Let’s take a look at how Eigen allows us to do this with simplicity and ease:

```Eigen::Matrix2f a;
Eigen::Vector2f v;
a << 1, 2,
3, 4;
v << 5,
6;
std::cout << "a * v =\n" << a * v << std::endl;```

This code creates a 2×2 matrix and a 2D vector, multiplies them together, and prints the result. The result, in this case, would be a 2D vector.

## Solving Linear Equations

Solving systems of linear equations is a vital task in numerous fields, including computer graphics, physics simulations, and machine learning. Eigen provides a range of methods to do just that:

```Eigen::Matrix2f A;
Eigen::Vector2f b;
A << 2, -1,
-1, 2;
b << 1,
2;
Eigen::Vector2f x = A.colPivHouseholderQr().solve(b);
std::cout << "The solution is:\n" << x << std::endl;```

This example defines a system of two linear equations, solves it, and prints the result.

## Matrix and Vector Transposition

A fundamental operation in linear algebra is the transposition of matrices and vectors. In Eigen, this task is realized briskly:

```Eigen::Matrix2f m;
m << 1, 2,
3, 4;

std::cout << "The transpose of m is:\n" << m.transpose() << std::endl;```

This example prints the transpose of the declared 2×2 matrix.

```Eigen::Vector2f v(1,2);
std::cout << "The transpose of v is:\n" << v.transpose() << std::endl;```

This example transposes 2D vectors.

## Matrix Determinant and Inverse

Determinants and inverses are important matrix operations used widely in simulations, image processing, and AI. Here’s how Eigen can compute these:

```Eigen::Matrix2f m;
m << 1, 2,
3, 4;
std::cout << "The determinant of m is:\n" << m.determinant() << std::endl;
std::cout << "The inverse of m is:\n" << m.inverse() << std::endl;```

This code calculates and prints both the determinant and inverse of a 2×2 matrix.

## Where to Go Next with C++?

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## Conclusion

That wraps up our exciting exploration of Eigen C++, a high-level C++ library for linear algebra. Whether you’re working on game physics or developing machine learning algorithms, Eigen offers a toolkit that enriches your programming abilities by facilitating intricate matrix and vector manipulations. Remember, any machine learning or game development task can be made easier with the right tools – and Eigen C++ is undoubtedly one of them!

Ready for further growth? Remember we’re here to guide you. Developing your skills with Zenva’s C++ Programming Academy will put you on a fast track to mastering C++ and unlocking your full potential. Get started on your journey today, and let Eigen C++ push your coding capabilities to new heights!

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