From: Ammar Ali <ammarabuali@...>
Date: 2010-11-13T01:18:32+09:00
Subject: Re: help sorting objects by their instance field

On Fri, Nov 12, 2010 at 5:43 PM, Aaron Haas <aaron4osu@gmail.com> wrote:
> I'm trying to figure out how to sort objects in an array by one of their
> instance
> variables, in this case last name. The person class has a first name,
> last name, and an address object. The address book class is made up of
> an array of persons. How do I sort the array of persons in
> alphabetically by the persons last name instance fields. I added a
> getLastname method to the persons class, but I'm not sure on the logic.
>
> [CODE]
> class Person
>    attr_accessor :fname, :lname, :address
>
>    # initializer
>    def initialize
>        @fname = @lname = ""
>        # since address is an object
>        @address = Address.new
>    end
>
>    # assign given values to Person object
>    def crPerson (aFname, aLname, aAddress)
>        @fname = aFname
>        @lname = aLname
>        @address = aAddress
>    end

To answer your main question first:

@persons.sort_by {|person| person.aLname}

Some notes:

Do you need to create blank Person objects? If not, you do the
initialization work of crPerson inside the initialize method.

def initialize(first_name, last_name, address)
  @fname = aFname
  @lname = aLname
  @address = aAddress
end

>    def get_lname
>        return @lname
>    end

If you use the attr_reader, attr_writer, and attr_accessor keywords,
you can do away with getter/setter methods. Example

class Person
  attr_accessor :first_name, :last_name
end

Now you can just write:

p = Person.new('Joe', 'Schmoe', '123 Main Street')
p.last_name # gets last name
p.first_name # gets first name

THe same applies for setting values:
p.last_name = 'Bloe'



>    # class variable. keeps track of number of address book entries
>    @@instances = 0

You can accomplish this with:

@persons.length

Class variables in ruby are rarely used, or what one thinks they need.


>     #  totally lost on this part
>    # print sorted by last name method using person's get_lname method
>    def print_addresses
>      @persons.each { |p| persons[p].get_lname.sort  }

First the sort is being applied to the last name. I doubt that's what
you want. Also, you don't need to index into the persons array here,
the p is the person you want already. This is the same:

@persons.each { |p| persons.last_name  }

I already addressed the sorting at the top of the post.

>      end # close while loop
>      # print sorted array
>      @persons.each { |p| yield p }
>    end

This doesn't print the persons, it just yields them. This does:

@persons.each {|p| puts p}

If you meant to use yield, then the printing is done in a block passed
to AddressBook class print_addresses method, so you need to add a
block argument and yield to it.

   def print_addresses(&block)
     @persons.sort_by { |p| persons.last_name  }
     @persons.each { |p| yield p }
   end

Then you use it like:

address_book.print_addresses {|p| puts p}

Note that I have used the common ruby naming convention for variables
and methos, last_name, instead of aLastName.

HTH,
Ammar