Sort_by Ruby，一降一升

Question

我搜索了一个无济于事的答案，有一个类似的问题，但答案在这种情况下不起作用，它按数字项排序。相似的问题-没用，我正在尝试使用ruby的sort_by对降序排列的项进行排序，而另一项则按升序进行排序。我所能找到的只是其中一个。

这是代码：

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

任何指导肯定会有所帮助。

样本数据：

输入

• 罗素，逻辑
• 欧拉，图论
• Galois，抽象代数
• 高斯数论
• 图灵算法
• 伽罗华逻辑

输出

• 图灵算法
• 罗素，逻辑
• 高斯数论
• Galois，抽象代数
• 伽罗华逻辑
• 欧拉，图论

Answer 1

这是一种简单的方法：

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"] ]

对于多个字段，首先按降序排序，然后依次按升序对其他每个字段排序：

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

<=>创建一个自定义类来反转（包括）的结果Comparable。

用自定义类包装您想要降序排序的对象。

例子：

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],
]}

输出：

{: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"}

顺便说一句，如果您想要降序排序的对象是数值，您可以简单地使用一元运算符-：

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