How can I sort by multiple conditions with different orders?

Question

I'd really like to handle this without monkey-patching but I haven't been able to find another option yet.

I have an array (in Ruby) that I need to sort by multiple conditions. I know how to use the sort method and I've used the trick on sorting using an array of options to sort by multiple conditions. However, in this case I need the first condition to sort ascending and the second to sort descending. For example:

ordered_list = [[1, 2], [1, 1], [2, 1]]

Any suggestions?

Edit: Just realized I should mention that I can't easily compare the first and second values (I'm actually working with object attributes here). So for a simple example it's more like:

ordered_list = [[1, "b"], [1, "a"], [2, "a"]]

Answer 1

How about:

ordered_list = [[1, "b"], [1, "a"], [2, "a"]]
ordered_list.sort! do |a,b|
  [a[0],b[1]] <=> [b[0], a[1]]
end

Answer 2

I was having a nightmare of a time trying to figure out how to reverse sort a specific attribute but normally sort the other two. Just a note about the sorting for those that come along after this and are confused by the |a,b| block syntax. You cannot use the {|a,b| a.blah <=> b.blah} block style with sort_by! or sort_by. It must be used with sort! or sort. Also, as indicated previously by the other posters swap a and b across the comparison operator <=> to reverse the sort order. Like this:

To sort by blah and craw normally, but sort by bleu in reverse order do this:

something.sort!{|a,b| [a.blah, b.bleu, a.craw] <=> [b.blah, a.bleu, b.craw]}

It is also possible to use the - sign with sort_by or sort_by! to do a reverse sort on numerals (as far as I am aware it only works on numbers so don't try it with strings as it just errors and kills the page).

Assume a.craw is an integer. For example:

something.sort_by!{|a| [a.blah, -a.craw, a.bleu]}

Answer 3

I had this same basic problem, and solved it by adding this:

class Inverter
  attr_reader :o

  def initialize(o)
    @o = o
  end

  def <=>(other)
    if @o.is && other.o.is
      -(@o <=> other.o)
    else
      @o <=> other.o
    end
  end
end

This is a wrapper that simply inverts the <=> function, which then allows you to do things like this:

your_objects.sort_by {|y| [y.prop1,Inverter.new(y.prop2)]}

Answer 4

Enumerable#multisort is a generic solution that can be applied to arrays of any size, not just those with 2 items. Arguments are booleans that indicate whether a specific field should be sorted ascending or descending (usage below):

items = [
  [3, "Britney"],
  [1, "Corin"],
  [2, "Cody"],
  [5, "Adam"],
  [1, "Sally"],
  [2, "Zack"],
  [5, "Betty"]
]

module Enumerable
  def multisort(*args)
    sort do |a, b|
      i, res = -1, 0
      res = a[i] <=> b[i] until !res.zero? or (i+=1) == a.size
      args[i] == false ? -res : res
    end
  end
end

items.multisort(true, false)
# => [[1, "Sally"], [1, "Corin"], [2, "Zack"], [2, "Cody"], [3, "Britney"], [5, "Betty"], [5, "Adam"]]
items.multisort(false, true)
# => [[5, "Adam"], [5, "Betty"], [3, "Britney"], [2, "Cody"], [2, "Zack"], [1, "Corin"], [1, "Sally"]]

Answer 5

I've been using Glenn's recipe for quite a while now. Tired of copying code from project to project over and over again, I've decided to make it a gem:

http://github.com/dadooda/invert