#### Overview of this book

Ruby is a powerful, general-purpose programming language that can be applied to any task. Whether you are an experienced developer who wants to learn a new language or you are new to programming, this book is your comprehensive Ruby coding guide. Starting with the foundational principles, such as syntax, and scaling up to advanced topics such as big data analysis, this book will give you all of the tools you need to be a professional Ruby developer. A few of the key topics are: object-oriented programming, built-in Ruby methods, core programming skills, and an introduction to the Ruby on Rails and Sinatra web frameworks. You will also build 10 practical Ruby programs. Created by an experienced Ruby developer, this book has been written to ensure it focuses on the skills you will need to be a professional Ruby developer. After you have read this book, you will be ready to start building real-world Ruby projects.
Preface
Free Chapter
Introduction to the Ruby Programming Language
Ruby Variables
Ruby Strings
Working with Numbers in Ruby
Ruby Iterators and Loops
Ruby Collections
Ruby Conditionals
Working with the Filesystem in Ruby
Error Handling in Ruby
Regular Expressions in Ruby
Searching with grep in Ruby
Ruby Gems
Ruby Web Frameworks
Working with APIs in Ruby
Machine Learning

# Implementing a permutation algorithm

In this section, we are going to build an algorithm that is going to leverage a number of powerful Ruby methods. The math problem that we are going to solve is a problem that asks this: what is the millionth lexicographic permutation of the digits 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9?

This may seem like an intimidating problem, but thankfully, it can be solved quickly with Ruby, thanks to some functional programming methods.

So what is a lexicographic permutation? It's the number of permutations you can make with a given set of numbers. For example, there are six different numbers you can create with the digits 0, 1, and 2.

Now that we know how to build a permutation for three numbers, we have to find the millionth permutation of the digits 0 to 9.

In Ruby, we can do this with a single line of code:

`p [0,1,2,3,4,5,6,7,8,9].permutation.to_a...`