#### 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 an amicable number algorithm

In this section, we are going to find a solution in Ruby for the math problem that asks this: evaluate the sum of all the amicable numbers under 10,000.

So that made perfect sense, right? In case your amicable number knowledge is a little rusty like mine was, let's look at a definition of amicable numbers:

Let d(n) be defined as the sum of proper divisors of n (numbers less than n that could be divided evenly into n). If d(a) = b and d(b) = a, where a ≠ b, then a and b are an amicable pair, and both a and b are called amicable numbers.

Still unclear? The following is the full explanation:

Essentially, you take a number, identify all its divisors, and add them together to get a value. Next, take that final sum value, identify its divisors, and add them together. If this second sum value is the same as the original number for...