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

Solution 1:

Perl was likely the first language to use it. Groovy is another language that supports it. Basically 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 useful for sorting an array.

Solution 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>]

Solution 3:

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.