Writing iterators

Writing good programs

More features

There are just a couple more things that we should do with our AddressBook class.

Private and public methods

Take a look at AddressBook#by_name. This method is different from the other methods in one important respect: It is used only inside the object. It is an internal method.

When you have a method like this, it is best to declare it as private. A private method can only be accessed by the object itself, and never by the user. A regular method, that is available to the user, is called public.

You can declare methods as public and private with the (surprise!) public and private keywords. When you put the keyword private you set all the methods defined from then on as private, until you change it back wit the public keyword.

class SomeClass

    def method1 # default to public
        ...
    end

  private   # subsequent methods are private.  

    def method2 # private method
        ...
    end
    def method3 # private method
        ...
    end

  public    # Set back to public.

    def method4 # public method
        ...
    end
end
                       

So, in our case, we just want to put the private keyword before we define the by_name method.

class AddressBook
    #
    #  Fundamental methods: initialize, add, remove
    #
    def initialize
        @persons = []
    end
    def add(person)
        @persons += [person]
        @persons = @persons.sort{|a,b| by_name(a,b)}  
    end

    ...

  private  # Start private methods
        
    #  
    #  Sorting function.
    #
    def by_name(a,b)
        if  a.first_name  == b.first_name
            a.last_name  <=> b.last_name
        else
            a.first_name <=> b.first_name
        end 
    end
end
                       

Code reuse with require

We have spent a lot of time writing these three classes. We don't want to copy and paste the code every time we want to use them in a program. Fortunatelly, we don't have to.

Put all three classes in a file, and save it as addressbook.rb. Now, create a new file (in the same directory) and type:

require "addressbook"

#  Sandy

addr = Address.new
addr.street = "324 Campus Dr."  
addr.city   = "College Park"
addr.state  = "MD"
addr.zip    = "23659"

puts addr
                       

Run the program, and you should get:

$ ruby prog.rb 
    324 Campus Dr.  
    College Park
    MD, 23659
$
                       

The require line lets you reuse your code on any other programs you write.

Writing iterators

Writing good programs