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Class variable: A variable that can be accessed and used within the scope of a class and all of its child objects. Encapsulation: The concept of allowing methods to have differing degrees of visibility outside of their class or associated object. Polymorphism: The concept of methods being able to deal with different classes of data and offering a more generic implementation (as with the area and perimeter methods offered by your Square and Triangle classes). Module: An organizational element that collects together any number of classes, methods, and constants into a single namespace. Namespace: A named element of organization that keeps classes, methods, and constants from clashing. Mix-in: A module that can mix its methods in to a class to extend that class s functionality. Enumerable: A mix-in module, provided as standard with Ruby, that implements iterators and list-related methods for other classes, such as collect, map, min, and max. Ruby uses this module by default with the Array and Hash classes. Comparable: A mix-in module, provided as standard with Ruby, that implements comparison operators (such as <, >, and ==) on classes that implement the generic comparison operator <=>.

Throughout the next several chapters, I ll assume you have a knowledge of how classes and objects work, and how the different scopes of variables (including local, global, object, and class variables) work.

n previous chapters we ve looked at and worked with Ruby from a low-level perspective by working directly with classes, objects, and functions. Each line of code we ve used in the small projects so far has been written specifically for that project from scratch. In this chapter, we ll look at how to build larger projects with Ruby, and how to reuse code written previously. Finally, we ll look at how to use code already written and prepared by other developers within your own applications so that you don t need to reinvent the wheel every time you create a new program. This chapter is about the bigger picture: dealing with projects and libraries.

As you become more familiar with Ruby and find more uses for it, it s likely that you ll want to move from writing single small programs (with fewer than 100 or so lines) to more complex applications and systems made up of multiple parts. Larger applications and systems therefore often become known as projects, and are managed in a different way than simple one-file scripts. The most common way to separate functionality in Ruby is to put different classes in different files. This gives you the ability to write classes that could be used in multiple projects simply by copying the file into your other project.

Consider this code: puts "This is a test".vowels.join('-') If you try to execute this code, you ll get an error complaining that the vowels method is not available for the "This is a test" object of class String. This is true because Ruby doesn t provide that method. Let s write an extension to the String class to provide it: class String def vowels self.scan(/[aeiou]/i) end end

If this definition were included in the same file as the prior puts code say, my_test.rb the result would be as follows: i-i-a-e In this case, you ve extended String with a vowels method that uses scan to return an array of all the vowels (the i option on the end makes the regular expression case-insensitive). However, you might want to write a number of methods to add to String that you d like to use in multiple programs. Rather than copy and paste the code each time, you can copy it out to a separate file and use the require command to load the external file into the current program. For example, put this code in a file called string_extensions.rb: class String def vowels self.scan(/[aeiou]/i) end end And put this code in a file called vowel_test.rb: require 'string_extensions' puts "This is a test".vowels.join('-') If you run vowel_test.rb, the expected result would appear onscreen. The first line, require 'string_extensions', simply loads in the string_extensions.rb file and processes it as if the code were local. This means that, in this case, the vowels method is available, all with a single line. As well as require, you can use load to load external source-code files into your program. For example, this code would seem to function identically to the preceding code: load 'string_extensions.rb' puts "This is a test".vowels.join('-')