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
[1,3,2] <=> [2,2,2]
Ruby will start comparing each element of both array from left hand side.
1
for left array is smaller than2
of right array. Hence left array is smaller than right array. Output will be-1
.[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.