From: Jason Creighton <androflux@...>
Date: 2003-08-07T11:44:29+09:00
Subject: Re: What's New and Shiny in Ruby 1.8.0?

On Thu, 7 Aug 2003 02:26:35 +0900
Ian Macdonald <ian@caliban.org> wrote:

> On Wed 06 Aug 2003 at 10:17:44 +0900, Harry Ohlsen wrote:
> 
> > This would be particularly useful if the data items in the enumeration were 
> > instances of more complex objects. Then, you could do things like:
> > 
> >   sorted = people.sort_by { |p| [p.last_name, p.first_name, p.salary] }
> 
> How does one use this with one's own objects, though?
> 
> I just cooked up an example Person class to play with and then tried to
> sort it as above:
> 
> class Person
>    include Enumerable
     ^^^^^^^^^^^^^^^^^^
Person doesn't implement #each, why mixin Enumerable?

>    attr_accessor :first_name, :last_name, :age
>  
>    def initialize(f,l,a)
>      @first_name = f
>      @last_name = l
>      @age = a.to_i
>    end
>  end
> 
> p [d, g, i, j, p, j2, j3].sort_by { |x| [x.last_name, x.first_name, x.age] }
> 
> and I get:
> 
> /sort_by:23: undefined method `[]' for #<Person:0x4014e37c>
> (NoMethodError)
> 
> I'm fine with implementing Person#[], but I can't imagine how it should
> look.

~/prog/ruby$ ruby -v
ruby 1.8.0 (2003-08-04) [i686-linux]
~/prog/ruby$ cat person.rb 
class Person
  attr_accessor :first_name, :last_name

  def initialize(f,l)
   @first_name = f
   @last_name = l
  end

  def inspect
    "#{@first_name} #{@last_name}"
  end
end

ary = [ Person.new("Joe", "Blow"), 
        Person.new("John", "Doe"),
        Person.new("Bill", "Blow"),
        Person.new("Jane", "Doe") ]
p ary.sort_by { |x| [x.last_name, x.first_name ] }
~/prog/ruby$ ruby person.rb 
[Bill Blow, Joe Blow, Jane Doe, John Doe]

BTW, your original example worked fine on my version of Ruby. Another
option, of course, is to implement <=> for Person like so (untested)

class Person
  def <=>(other)
    [ @last_name, @first_name ] <=> [ other.last_name, other.first_name ]
  end
end

Jason Creighton