How to pass a custom comparator to "sort"?

Question

Class A has the following comparator:

class A   attr_accessor x    def my_comparator(a)     x**2 <=> (a.x)**2   end end 

I would like to use this comparator to sort an array where each item is of class A:

class B   def my_method     items.sort!(<how can I pass my_comparator here ?>)   end end 

How should I pass my_comparator to sort!?

Answer 1

Define your own <=>, and include Comparable. This is from the Comparable doc:

class SizeMatters   include Comparable   attr :str   def <=>(an_other)     str.size <=> an_other.str.size   end   def initialize(str)     @str = str   end   def inspect     @str   end end  s1 = SizeMatters.new("Z") s2 = SizeMatters.new("YY") s3 = SizeMatters.new("XXX") s4 = SizeMatters.new("WWWW") s5 = SizeMatters.new("VVVVV")  s1 < s2                       #=> true s4.between?(s1, s3)           #=> false s4.between?(s3, s5)           #=> true [ s3, s2, s5, s4, s1 ].sort   #=> [Z, YY, XXX, WWWW, VVVVV] 

You don't actually have to include Comparable, but you get extra functionality for free if you do that after having defined <=>.

Otherwise, you can use Enumerable's sort with a block if your objects implement <=> already.

Another way to use several different comparisons is to use lambdas. This uses the new 1.9.2 declaration syntax:

ascending_sort  = ->(a,b) { a <=> b } descending_sort = ->(a,b) { b <=> a }  [1, 3, 2, 4].sort( & ascending_sort ) # => [1, 2, 3, 4] [1, 3, 2, 4].sort( & descending_sort ) # => [4, 3, 2, 1]  foo = ascending_sort [1, 3, 2, 4].sort( & foo ) # => [1, 2, 3, 4] 

Answer 2

Both of these should work:

items.sort_by! { |a| (a.x)**2 } items.sort! { |a1,a2| a1.my_comparator(a2) }