Ruby Hash Example

ruby_hash_example.rb

http://blog.sandbochs.com
# The hash method produces a different value
# every time you load up ruby
# because it is based off of the object_id
1.hash => 3843363811812838534
num = 1
num.hash => 3843363811812838534

# Strings override Object's hash method
# The hash function is based on the characters in the string
"apples".hash => 1951414395381147893 
string = "apples"
string.hash => 1951414395381147893 
"Apples".hash => -3796558981278204666 

# A new Apple class where all apples weigh 6 oz
class Apple
  def initialize
    @weight = 6
  end
end

a1 = Apple.new
a2 = Apple.new

# The hash methods produce different values
a1.hash => -3674294693357128426 
a2.hash => 3276174667199791386

directory = {a1 => "apple shack"}
directory[a1] => "apple shack"
directory[a2] => nil

# Let's override the hash function
class Apple
  def hash
    5 * @weight
  end
end

# The hash methods produce the same hash
# for apples that are the same weight
a1.hash => 30
a2.hash => 30
a1 == a2 => false

# Still nil though...
directory = {a1 => "apple shack"}
directory[a1] => "apple shack"
directory[a2] => nil

# Right, the hash table will compare the object
# you are looking up with the object
# inside the bucket! Override == operator...
class Apple
  attr_reader :weight
  def ==(other)
    weight == other.weight
  end
end

a1.hash => 30
a2.hash => 30
a1 == a2 => true

# What?! still nil!
directory[a1] => "apple shack"
directory[a2] => nil

# Ruby's implementation of hashes actually
# compares with the eql? method, not ==
class Apple
  def eql?(other)
    self == other
  end
end

a1.hash => 30
a2.hash => 30
a1.eql? a2 => true

# COOL HUH!?
directory[a1] => "apple shack"
directory[a2] => "apple shack"

Nice example!