Sort_by Ruby, One Descending, One Ascending

Question

I have searched for an answer on this to no avail, there is a similar question but the answer did not work in this situation, it sorts on a numeric item. Similar Question -That did not work I am trying to use ruby's sort_by to sort one item descending with the other ascending. All I can find is one or the other.

Here is the code:

# Primary sort Last Name Descending, with ties broken by sorting Area of interest.
people = people.sort_by { |a| [ a.last_name, a.area_interest]}

Any guidance would certainly assist.

Sample data:

input

• Russell, Logic
• Euler, Graph Theory
• Galois, Abstract Algebra
• Gauss, Number Theory
• Turing, Algorithms
• Galois, Logic

output

• Turing , Algorithms
• Russell, Logic
• Gauss, Number Theory
• Galois, Abstract Algebra
• Galois, Logic
• Euler, Graph Theory

Answer 1

This is a straightforward way:

a = [ ['Russell', 'Logic'],           ['Euler', 'Graph Theory'],
      ['Galois', 'Abstract Algebra'], ['Gauss', 'Number Theory'],
      ['Turing', 'Algorithms'],       ['Galois', 'Logic'] ]

a.sort { |(name1,field1),(name2,field2)|
  (name1 == name2) ? field1 <=> field2 : name2 <=> name1 }
#=> [ ["Turing", "Algorithms"],   ["Russell", "Logic"],
#     ["Gauss", "Number Theory"], ["Galois", "Abstract Algebra"],
#     ["Galois", "Logic"],        ["Euler", "Graph Theory"] ]

For multiple fields, sorting on the first in descending order, then on each of the others, in sequence, in ascending order:

a = [ %w{a b c}, %w{b a d}, %w{a b d}, %w{b c a}, %w{a b c}, %w{b c b}]
  #=> [["a", "b", "c"], ["b", "a", "d"], ["a", "b", "d"],
  #    ["b", "c", "a"], ["a", "b", "c"], ["b", "c", "b"]] 

a.sort { |e,f| e.first == f.first ? e[1..-1] <=> f[1..-1] : f <=> e }
  #=> [["b", "a", "d"], ["b", "c", "a"], ["b", "c", "b"],
  #    ["a", "b", "c"], ["a", "b", "c"], ["a", "b", "d"]] 

Answer 2

Make a custom class that invert the result of <=> (including Comparable).

Wrap the object you want sort descending with the custom class.

Example:

class Descending
  include Comparable
  attr :obj

  def initialize(obj)
    @obj = obj
  end
  def <=>(other)
    return -(self.obj <=> other.obj)
  end
end

people = [
  {last_name: 'Russell', area_interest: 'Logic'},
  {last_name: 'Euler', area_interest: 'Graph Theory'},
  {last_name: 'Galois', area_interest: 'Abstract Algebra'},
  {last_name: 'Gauss', area_interest: 'Number Theory'},
  {last_name: 'Turing', area_interest: 'Algorithms'},
  {last_name: 'Galois', area_interest: 'Logic'},
]
puts people.sort_by {|person| [
  Descending.new(person[:last_name]),  # <---------
  person[:area_interest],
]}

output:

{:last_name=>"Turing", :area_interest=>"Algorithms"}
{:last_name=>"Russell", :area_interest=>"Logic"}
{:last_name=>"Gauss", :area_interest=>"Number Theory"}
{:last_name=>"Galois", :area_interest=>"Abstract Algebra"}
{:last_name=>"Galois", :area_interest=>"Logic"}
{:last_name=>"Euler", :area_interest=>"Graph Theory"}

BTW, if the object you want sort descending is a numeric value, you can simply use unary operator -:

people.sort_by {|person| [-person.age, person.name] }