From: Alex Young <alex@...>
Date: 2007-03-28T01:16:56+09:00
Subject: Re: sorting an array based on two attributes of objects

Rick DeNatale wrote:
> On 3/26/07, Alex Young <alex@blackkettle.org> wrote:
<snip>
>> In this case, just invert the salary:
>>
>> sorted_employees = employees.sort_by { |e| [ -e.salary, e.name ] }
>>
>> I can't think of a way offhand to have different lexicographic orderings
>> in the same sort, though.
> 
> Someone, in another recent thread, came up with the idea of a reverse
> proxy class something like this
> 
> class Reverse
> 
>   :attr_reader :obj
> 
>   def initialize(obj)
>      @obj = obj
>   end
> 
>   def <=>(other)
>       other.obj <=> self.obj
>   end
> 
> end
> 
> so let's say you wanted to sort the names in descending order as well:
> sorted_employees = employees.sort_by { |e| [-e.salary, 
> Reverse.new(e.name) ] }
> 
> You could even use this "reverse" proxy to change the sorting of
> salary, so that the original could be:
> 
> sorted_employees = employees.sort_by { |e| [ Reverse.new(e.salary), 
> e.name ] }

That's lovely.  My brain wandered off down the String#invert route, and 
I ended up getting tied in knots over multibyte encodings.  Reverse is a 
*much* nicer trick.

-- 
Alex