From: Rick DeNatale <rick.denatale@...>
Date: 2007-03-28T01:10:13+09:00
Subject: Re: sorting an array based on two attributes of objects

On 3/26/07, Alex Young <alex@blackkettle.org> wrote:
> senthil wrote:
> >  Thanks for some of your replies for the lst post i need small
> > modification in that post .. i want to sort the salary in descending
> > order. i have explained the same quesion again....
> >
> > Hi all,
> >      I want to sort the objects of array based on two attributes.I want
> > sort an employee class based on his salary in descecding order and name
> > , so that if two
> > person has same salary it should be sorted with name.
> >      Lets say for example the employee objects has following name and
> > salary.
> >
> >          name      salary
> >            d        100
> >            c        200
> >            b        50
> >            a        100
> >
> >
> >      so in this case the result which i expect is
> >
> >          name      salary
> >            b        200
> >            a        100
> >            d        100
> >            c        50
> >
> >   Note:In the above example for salary 100 the sorting is done
> > alphabetically, but initially(before sorting) 'd' came first and then
> > 'a' came.so basically i want to sort the array with more than one order.
> >   Can any one help me to solve it ??
> >
> 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 ] }

One could quibble about the name Reverse.

-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/