From: Rob Biedenharn <Rob@...>
Date: 2011-07-01T21:24:23+09:00
Subject: Re: How to order Structs based on two fields

On Jul 1, 2011, at 7:18 AM, Robert Klemme wrote:

> On Fri, Jul 1, 2011 at 12:55 PM, Iņaki Baz Castillo <ibc@aliax.net>  
> wrote:
>> 2011/7/1 Iņaki Baz Castillo <ibc@aliax.net>:
>>> I expected that the following could work:
>>>
>>>  array.sort_by{|entry| entry.a or entry.b}
>>>
>>> but it generates:
>>>
>>>   [kk1, kk2, kk3, kk4]
>>>
>>> so obviously it does not work. I'm not very used to  
>>> Enumerable.sort_by
>>> method, any tip please?
>>
>> Maybe it would be better to use "class KK" rather than an struct, and
>> then define <=> method for KK class (and include Enumerable module)?
>
> You can do
>
> array.sort_by {|e| [e.a, e.b]}
>
> But you can of course define <=> for a Struct class as well
>
> KK = Struct.new :a,:b do
>  include Comparable
>
>  def <=>(o)
>    cmp = a <=> o.a
>    cmp == 0 ? b <=> o.b : cmp
>  end
> end

Actually, there's a method to help with exactly this sort of thing  
(pun intended).

def <=>(o)
   (a <=> o.a).nonzero? || b <=> o.b
end

Take a look at Numeric#nonzero? and you'll find an example that's  
almost exactly this.

-Rob

>
> Kind regards
>
> robert
>
> -- 
> remember.guy do |as, often| as.you_can - without end
> http://blog.rubybestpractices.com/
>

Rob Biedenharn		
Rob@AgileConsultingLLC.com	http://AgileConsultingLLC.com/
rab@GaslightSoftware.com		http://GaslightSoftware.com/