What is the Ruby <=> (spaceship) operator?

Question

What is the Ruby <=> (spaceship) operator? Is the operator implemented by any other languages?

Answer 1

It's also known as the Three-Way Comparison Operator. Perl was likely the first language to use it. Some other languages that support it are: Apache Groovy, PHP 7+, and C++20.

Instead of returning 1 (true) or 0 (false) depending on whether the arguments are equal or unequal, the spaceship operator will return 1, 0, or −1 depending on the value of the left argument relative to the right argument.

a <=> b :=   if a < b then return -1   if a = b then return  0   if a > b then return  1   if a and b are not comparable then return nil 

It's commonly used for sorting data.

Answer 2

The spaceship method is useful when you define it in your own class and include the Comparable module. Your class then gets the >, < , >=, <=, ==, and between? methods for free.

class Card   include Comparable   attr_reader :value    def initialize(value)     @value = value   end    def <=> (other) #1 if self>other; 0 if self==other; -1 if self<other     self.value <=> other.value   end  end  a = Card.new(7) b = Card.new(10) c = Card.new(8)  puts a > b # false puts c.between?(a,b) # true  # Array#sort uses <=> : p [a,b,c].sort # [#<Card:0x0000000242d298 @value=7>, #<Card:0x0000000242d248 @value=8>, #<Card:0x0000000242d270 @value=10>]