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Eureka! You ll notice that the way you make the dog bark is simply by referring to the dog (a_dog, in this case) and putting a period ( . ) followed by the bark method s name, whereupon your dog barks. Let s dissect exactly what happened. First, you added a bark method to your Dog class. The way you did this was by defining the method. To define a method, you use the word def followed by the name of the method you wish to define. This is what the def bark line means. It means I m defining the bark method within this class until I say end. The following line then simply puts the word Woof! on the screen, and the last line of the method ends the definition of that method. The last end ends the class definition (this is why indentation is useful, so you can see which end lines up with which definition). The Dog class then contains a new method called bark, as you used earlier. Think about how you would create methods for the other Pet classes or the Pet class itself. Are there any methods that are generic to all pets If so, they d go in the Pet class. Are there methods specific to cats They d go in the Cat class.

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In this chapter we ve looked at how Ruby can understand concepts in the form of classes and objects. We created virtual cats and dogs, gave them names, and triggered their methods (the bark method, for example). These basic concepts form the core of objectoriented programming, and you ll use them constantly throughout this book. Dogs and cats are merely an example of the flexibility object orientation offers, but the concepts we ve used so far could apply to most concepts, whether we re giving a ticket a command to change its price or a user a command to change his or her password. Begin to think of the programs you want to develop in terms of their general concepts and how you can turn them into classes you can manipulate with Ruby. Among even object-oriented programming languages, Ruby is reasonably unique in that almost everything in the language is an object, even the concepts relating to the language itself. Consider the following line of code:

than None).

After you master the fundamentals of the BBP approach, you can customize the settings of the BBP Storyboard Formatter according to your needs. You can also save the template to your local computer to make it easily accessible when you create a new presentation; for details, see the section Tip 3: Install the Storyboard Formatter on Your Local Computer later in this chapter.

If you typed this into irb and pressed Enter, you d see the number 11 in response. You ve asked Ruby to print the result of 1 + 10 to the screen. It seems simple enough, but believe it or not, this simple line uses two objects. 1 is an object, as is 10. They re objects of class Fixnum, and this built-in class has methods already defined to perform operations upon numbers, such as addition and subtraction. We ve considered how concepts can be related to different classes. Our pets make a good example. However, even defining the concepts that programmers use to write computer programs as classes and objects makes sense. When you write a simple sum such as 2 + 2, you expect the computer to add two numbers together to make 4. In its objectoriented way, Ruby considers the two numbers in the sum (2 and 2) to be number objects. 2 + 2 is merely shorthand for asking the first number object to add the second number object to itself. In fact, the + sign is actually an addition method! You can prove that everything in Ruby is an object by asking things of which class they re a member. In the pet example earlier, you could have made a_dog tell you what class it s a member of with the following code:

It s worth noting that you could achieve the same results as the preceding results by placing the expressions outside the strings, without using interpolation. For example:

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x = 10 y = 20 puts x.to_s + " + " + y.to_s + " = " + (x + y).to_s puts "#{x} + #{y} = #{x + y}"

The two puts lines result in the same output. The first uses string concatenation (+) to join several different strings together. The numbers in x and y are converted to strings with their to_s method. However, the second puts line uses interpolation, which doesn t require the numbers to be converted to strings explicitly.

Using pop, you can implement a common data structure called a stack. A stack like this works just like a stack of plates. You can put plates on top, and you can remove plates from the top. The last one you put into the stack is the first one to be removed. (This principle is called Last-In, First-Out, or LIFO.)