Ruby background: What are linked lists? - ruby

Question

I'm a self taught programmer and started off with Ruby. I'm currently trying to learn algorithms and data structures and noticed something called a linked list. It looks like Ruby doesn't have that data structure (maybe in Array?). I'm familiar with arrays and hashes.
How would you describe/explain what a linked list is for someone coming from my background?

Answer 1

Linked lists are rather simple: they are lists which are created by linking the elements together. (Kind of obvious, when you think about it, no?)
In its most basic form, a linked list is simply either empty or a pair. The first element of the pair is a value, and the second element of the pair is a linked list representing the rest of the values.
Traditionally, the pair is called a cons cell, the first element is called the head or the car of the list and the second element is called the tail or the cdr of the list. The empty list is called Nil or simply [].
Here is an extremely simple implementation of a linked list in Ruby, which uses nothing but functions to implement a linked list (and booleans and conditionals, while we're at it):
True = ->(iff, _) { iff }
False = ->(_, els) { els }
Pair = ->(first, rest) { -> x { x.(first, rest) }}
First = -> list { list.(True ) }
Rest = -> list { list.(False) }
Here, we have a simple list with three elements:
List = Pair.(1, Pair.(2, Pair.(3, nil)))
First.(Rest.(List))
# => 2
A more realistic object-oriented encoding of a list looks like this:
class List
def cons(el) Pair.new(el, self) end
Empty = new
class Pair < self
attr_reader :first, :rest
def initialize(first, rest=Empty)
self.first, self.rest = first, rest
end
private
attr_writer :first, :rest
end
end
Again, a list with two elements:
list1 = List::Pair.new(1, List::Pair.new(2, List::Pair.new(3, List::Empty)))
# corresponds to the list [1, 2, 3]
list1.rest.first
# => 2
list2 = List::Empty.cons(6).cons(5).cons(4)
# List[4, 5, 6]
list2.rest.first
# => 5
A more complete and Rubyish implementation might look like this:
class List
include Enumerable
def self.[](*els) els.reverse_each.inject(Empty, &:cons) end
def cons(el) Pair[el, self] end
def prepend(prefix)
case
when empty? then prefix
when prefix.empty? then self
else prepend(prefix.rest).cons(prefix.first)
end
end
def to_s; "List[#{map(&:to_s).join(', ')}]" end
def inspect; "List[#{map(&:inspect).join(', ')}]" end
def each; return enum_for(__method__) unless block_given? end
class << Empty = new
def empty?; true end
alias_method :inspect, def to_s; 'Empty' end
freeze
end
Empty.freeze
class Pair < self
def initialize(first, rest=Empty)
self.first, self.rest = first, rest
freeze
end
def empty?; false end
def each(&blk)
return super unless block_given?
yield first
rest.each(&blk)
end
private
attr_writer :first, :rest
protected
attr_reader :first, :rest
class << self; alias_method :[], :new end
freeze
end
freeze
end
Some examples:
list1 = List::Pair[1, List::Pair[2, List::Pair[3, List::Empty]]]
# => List[1, 2, 3]
list2 = List::Empty.cons(6).cons(5).cons(4)
# => List[4, 5, 6]
list3 = List[7, 8, 9]
# => List[7, 8, 9]
list4 = list3.prepend(list2).prepend(list1)
# => List[1, 2, 3, 4, 5, 6, 7, 8, 9]
list4.partition(&:odd?)
# => [[1, 3, 5, 7, 9], [2, 4, 6, 8]]

Answer 2

In computer science, a linked list is a data structure consisting of a group of nodes which together represent a sequence. Under the simplest form, each node is composed of a data and a reference (in other words, a link) to the next node in the sequence; more complex variants add additional links. This structure allows for efficient insertion or removal of elements from any position in the sequence.
http://en.wikipedia.org/wiki/Linked_list
Here is a implementation on ruby: http://matt.weppler.me/2013/08/14/implementing-a-linked-list-in-ruby.html
Kudos for studying Ruby one of my favorites languages.