Writing iterators

In this section we are going to add our very own iterators to the class AddressBook. We will create two iterators, AddressBook#each and AddressBook#each_address, which can be used as usual:


address_book.each do |person|
    ...
end

address_book.each_address do |address|  
    ...
end

Calling a block

When you write a function or method, you can call a code block with the yield keyword. Here is an example.


def twice
    yield
    yield
end

twice { puts "Hello World" }

This prints:


Hello World
Hello World

Passing parameters

You can use yield like any other method. To pass parameters to the code block, you simply pass them to yield. Take this example:


def names
    yield("Joe")
    yield("Sandy")
    yield("Melissa")
end

names do |name|
    puts "Hello " + name + ", how are you?"  
end

This prints:


Hello Joe, how are you?  
Hello Sandy, how are you?
Hello Melissa, how are you?

You can pass as many parameters to the block as you like. For instance:


def full_names
    yield("Joe", "Smith")
end

full_names do |first,last|
    puts first + " " + last  
end

Prints:


Joe Smith

AddressBook#each

Now we can write our first iterator. AddressBook#each is the simplest of the two iterators. We step through every person in the array @persons and call yield on each one.


class AddressBook
    def each
        @persons.each { |p| yield p }  
    end
end

That's it. Now your class has an iterator.

AddressBook#each_address

This iterator is almost as simple as the one we just wrote. We go through each person and pass in their address.


class AddressBook
    def each_address
        @persons.each { |p| yield p.address }  
    end
end

Wrap up

Just to wrap up everything up to now. Here is the entire class AddressBook together with comments. It is a fairly complex piece of code, but by splitting the task into smaller parts we have made it more manageable.


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
    def remove(person)   
        @persons.delete(person)
    end
     

    #
    #  Iterators:  each, each_address
    #
    def each
        @persons.each { |p| yield p }
    end
    def each_address
        @persons.each { |p| yield p.address }
    end
        
    #  
    #  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

Implementing AddressBook

More features