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

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>]

I will explain with simple example

1. [1,3,2] <=> [2,2,2]

   Ruby will start comparing each element of both array from left hand side. 1 for left array is smaller than 2 of right array. Hence left array is smaller than right array. Output will be -1.

2. [2,3,2] <=> [2,2,2]

   As above it will first compare first element which are equal then it will compare second element, in this case second element of left array is greater hence output is 1.

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.

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.

It's a general comparison operator. It returns either a -1, 0, or +1 depending on whether its receiver is less than, equal to, or greater than its argument.