Single Linked List Implementation in Ruby

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

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  ")
end

StackExchange Code Review Q#152732, answer score: 4