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One common feature used in Ruby is the ability to include code situated in other files into the current program (this is covered in depth in the next chapter). When including other files, you can quickly run into conflicts, particularly if files or libraries you re including then include multiple files of their own. You cannot guarantee that no file that s included (or one that s included in a long chain of includes) will clash with code you ve already written or processed.
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Take this example: def random rand(1000000) end puts random The random method returns a random number between 0 and 999,999. This method could be in a remote file where it s easily forgotten, which would cause problems if you had another file you included using require that implemented a method like so: def random (rand(26) + 65).chr end This random method returns a random capital letter.
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Note (rand(26) + 65).chr generates a random number between 0 and 25 and adds 65 to it, giving a number in the range of 65 to 90. The chr method then converts a number into a character using the ASCII standard where 65 is A, through to 90, which is Z. You can learn more about the ASCII character set at http://en.wikipedia.org/wiki/ASCII, or refer to 3, where this topic was covered in more detail.
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Now you have two methods called random. If the first random method is in a file called number_stuff.rb and the second random method is in a file called letter_stuff.rb, you re going to hit problems: require 'number_stuff' require 'letter_stuff' puts random Which version of the random method is called
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require is a Ruby statement used to load in code contained within another file. This is covered in
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As the last file loaded, it turns out to be the latter version of random, and a random letter should appear onscreen. Unfortunately, however, it means your other random method has been lost. This situation is known as a name conflict, and it can happen in even more gruesome situations than the simplistic example shown in the preceding code. For example, class names
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can clash similarly, and you could end up with two classes mixed into one by accident. If a class called Song is defined in one external file and then defined in a second external file, the class Song available in your program will be a dirty mix of the two. Sometimes this might be the intended behavior, but in other cases this can cause significant problems. Modules help to solve these conflicts by providing namespaces that can contain any number of classes, methods, and constants, and allow you to address them directly. For example: module NumberStuff def NumberStuff.random rand(1000000) end end module LetterStuff def LetterStuff.random (rand(26) + 65).chr end end puts NumberStuff.random puts LetterStuff.random
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Note Due to the randomness introduced by using rand, the results will vary every time you run the
program!
In this demonstration it s clear which version of random you re trying to use in the two last lines. The modules defined in the preceding code look a little like classes, except they re defined with the word module instead of class. However, in reality you cannot define instances of a module, as they re not actually classes, nor can they inherit from anything. Modules simply provide ways to organize methods, classes, and constants into separate namespaces. A more complex example could involve demonstrating two classes with the same name, but in different modules: module ToolBox class Ruler attr_accessor :length end end
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module Country class Ruler attr_accessor :name end end a = ToolBox::Ruler.new a.length = 50 b = Country::Ruler.new b.name = "Ghengis Khan from Moskau" Rather than having the Ruler classes fighting it out for supremacy, or ending up with a mutant Ruler class with both name and length attributes (how many measuring rulers have names ), the Ruler classes are kept separately in the ToolBox and Country namespaces. You ll be looking at why namespaces are even more useful than this later, but first you have to look at the second reason why modules are so useful.
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