Single Linked List Implementation in Ruby

Question

The #traverse_until method is a helper function that traverses the linked list and returns the node and index of the node once a condition (in a block) is met or until the end of the list. I would greatly appreciate feedback about verbosity, efficiency, readability, etc.

module DataStructures
 class Node
 attr_accessor :value, :next_node
 def initialize(value = nil, next_node = nil)
 @value = value
 @next_node = next_node
 end
 end
 class LinkedList
 attr_accessor :head
 def initialize(element = nil)
 element.nil? ? @head = nil : @head = Node.new(element)
 end
 def append(data)
 prepend(data) if @head.nil?
 current = @head
 until current.next_node.nil?
 current = current.next_node
 end
 current.next_node = Node.new(data)
 end
 def prepend(data)
 @head = Node.new(data, @head)
 end
 def find(data)
 result = traverse_until { |node, index| node.value == data }
 result[:node].value == data ? result[:index] : "Data not found."
 end
 def contains? (data)
 result = traverse_until { |node, index| node.value == data }
 result[:node].value == data
 end
 def node_at_index(index)
 result = traverse_until { |node, i| i == index }
 "Node at index #{index}: #{result[:node].value}"
 end
 def head
 @head.nil? ? "Empty list" : @head.value
 end
 def tail
 last_node = traverse_until { |node, index| node.next_node.nil? }
 last_node[:node].value
 end
 def size
 last_node = traverse_until { |node, index| node.next_node.nil? }
 "List Size: #{last_node[:index] + 1}"
 end
 def insert_at(data, index)
 current = @head
 (index - 1).times do |n|
 current = current.next_node
 end
 node_shifted_right = current.next_node
 current.next_node = Node.new(data, node_shifted_right)
 end
 def remove_at(index)
 node_after_index = @head
 previous = node_after_index
 (index + 1).times do |n|
 node_after_index = node_after_index.next_node
 end
 (index - 1).times do |n|
 previous = previous.next_node
 end
 previous.next_node = node_after_index
 end
 def pop
 node_before = nil
 current = @head
 until current.next_node.nil?
 node_before = current
 current = current.next_node
 end
 node_before.next_node = nil
 end
 def to_s
 result = ""
 current = @head
 loop do
 result += "(#{current.value}) -> "
 break if current.next_node.nil?
 current = current.next_node
 end
 result += "nil"
 end
 private
 def traverse_until
 current = @head
 index = 0
 until current.next_node.nil? || yield(current, index)
 current = current.next_node
 index += 1
 end
 {node: current, index: index}
 end
 end
end

Answer 1

Firstly, your code's broken. If you instantiate a list with no initial value, and then append to it, it creates two nodes:

list = LinkedList.new
list.append("foo")
list.size #=> "List size: 2"

Secondly, implement an #each method to iterate through values and include Enumerable. That'll give a ton of methods for free: #find, #count, #to_a, and many others. Something like this should do:

def each
 node = @head
 until node.nil?
 yield node.value
 node = node.next_node
 end
end

Thirdly, return useful values. #size should not return a string. Neither should #head, #node_at_index, or #find. #size should return an int, and the others should return nil. Never ever return easy-to-print strings unless that's explicitly called for. Currently, you can get puzzling stuff like this:

some_list.head #=> "Empty list" (the head's value is the string "Empty list")
some_list.size #=> "List size: 2" (so, no, not empty)

And you can get useless stuff like this:

if some_array.size < some_list.size # ArgumentError: comparison of Fixnum with String failed
 ...

I trust you see the problem here.

Speaking of return values, you seem to have taken some care that your methods don't return a raw Node instance (which is smart, since nodes are internal/private to the linked-list)... but you made @head accessible from anywhere with attr_accessor. So some_list.head = "foo" is perfectly legal, and will break things. Also, #append and #prepend implicitly return Node instances.

Fourth, you might want to keep track of the tail as well as the head. Many of your methods rely on finding the tail one way of another, so why not keep it around? Or at least use your own #tail method for these cases, rather than do it differently each time.

Other stuff:

• Check the indices passed to #insert_at and #remove_at and raise an appropriate error if the index is out-of-bounds. Otherwise you'll just get a NoMethodError which doesn't explain much.

• Don't put assignments into ternary branches; use the branch to determine the value to assign. I.e. this

  element.nil? ? @head = nil : @head = Node.new(element)
  

  should be

  @head = element.nil? ? nil : Node.new(element)
  

  of course, in this particular case, @head will be nil by default, do you could just as well do:

  @head = Node.new(element) unless element.nil?
  

• Your #to_s would be a lot cleaner if you collected all the values in an array, used map to add parentheses, and join to add the "->" arrows. If you include Enumerable as described above, your #to_s could just be:

  def to_s
   values = map { |value| "(#{value})" }
   values << "nil"
   values.join(" -> ")
  end
  

• Adrian DeRose: Thank you, based on my code sample above, what would be a rough estimate of the gap between my current skill level and the level of skill expected of an entry level junior developer? I know I still have a while to go, but I am just looking for a metric of some sort. Again, thank you.
• Flambino: @AdrianDeRose That depends; different places have different ideas of "entry level/junior dev". Yes, you have some way to go, but you're def on your way. But I'd encourage you work on actual projects, like a gem or something. Find something to contribute to or build a Rails site, etc.. Implementing a linked list in Ruby is a somewhat pointless exercise; just use Array. Or get a gem for it instead of reinventing the wheel. Code challenges are like crosswords: Solving them won't make you a writer; writing will. Also, read more Ruby code; dig around in popular, proven code and learn from it
• Jörg W Mittag: @AdrianDeRose: In particular, the Prawn PDF library was (at least originally) specifically created for reading the code as an example of well-written, well-factored, well-designed, idiomatic, readable, beautiful Ruby and OO code. Prawn was written by Gregory Brown who is also the author of the Ruby Best Practices book and the Practicing Ruby blog.