From: "Y. NOBUOKA" <nobuoka@...>
Date: 2010-10-13T18:08:44+09:00
Subject: Re: sort_by: multiple fields with reverse sort

The one way to do a reverse sort is wrapping a sort target in a class
for a reverse sort.
For instance:

# This class is a class for a reverse sort.
# An Object of various datatypes can be wrapped in an object of this class.
class ItemForReverseSort
  def initialize( item )
    @item = item
  end
  def item
    @item
  end
  def <=>( target )
    ( self.item <=> target.item ) * (-1)
  end
end

b = [ ["radio", 30, 5], ["radio", 20, 5], ["archie", 20, 5], ["newton", 10, 3] ]

# normal sort
p b.sort_by{|x| [ x[1], x[0] ]}
  #=> [["newton", 10, 3], ["archie", 20, 5], ["radio", 20, 5], ["radio", 30, 5]]

# normal sort for 1st item, and reverse sort for 2nd item
p b.sort_by{|x| [ x[1], ItemForReverseSort.new(x[0]) ]}
  #=> [["newton", 10, 3], ["radio", 20, 5], ["archie", 20, 5], ["radio", 30, 5]]


2010/10/13 Rahul Kumar <sentinel1879@gmail.com>:
> I need to use *sort_by* to sort a table, since the user could select
> columns in any order.
>
>    b=[ ["radio", 30, 5], ["radio", 20, 5], ["archie", 20, 5],
> ["newton", 10, 3] ]
>    b.sort_by{|x| [ x[1], x[0] ]}
>
> Works fine. However, often the user will want a reverse sort on some
> field. I do not off-hand know the datatype of the field, but in most
> cases it will be a String.
>
> I tried:
>
>    b.sort_by{|x| [ !x[1] ]}
> This works (one criteria), but this does not:
>    b.sort_by{|x| [ !x[1], x[0] ]}
>
> Another thread here, suggested using a minus for Numbers but what about
> Strings.
> Thx, rahul.

-- 
NOBUOKA Yuya