Custom Collections

Part of the Object Oriented Programming section of Coddy's Ruby journey. Lesson 40 of 57.

A collection class combines several special methods: each with Enumerable, << for adding, size, and to_s and inspect for printing. each should also work without a block, returning an enumerator with to_enum, as arrays do:

class TodoList
  include Enumerable

  def initialize
    @items = []
  end

  def <<(item)
    @items << item
    self
  end

  def each
    return to_enum(:each) unless block_given?

    @items.each { |item| yield item }
    self
  end
end

list = TodoList.new << "wash" << "shop" << "cook"
list.each.with_index(1) { |item, n| puts "#{n}. #{item}" }
p list.select { |i| i.include?("o") }

Output:

1. wash
2. shop
3. cook
["shop", "cook"]

block_given? tells whether the caller passed a block. The enumerator returned without one supports chaining such as each.with_index, each_slice or next:

class Countdown
  include Enumerable

  def initialize(from)
    @from = from
  end

  def each
    return to_enum(:each) unless block_given?

    @from.downto(1) { |n| yield n }
  end
end

e = Countdown.new(3).each
puts e.next
puts e.next
p Countdown.new(5).each_slice(2).to_a

Output:

3
2
[[5, 4], [3, 2], [1]]

to_s is the friendly text for users and inspect a short summary for programmers. For a collection, inspect usually shows the class and the size rather than every element:

class TodoList
  include Enumerable

  def initialize(*items)
    @items = items
  end

  def each(&block)
    @items.each(&block)
  end

  def size
    @items.size
  end

  def to_s
    map.with_index(1) { |item, n| "#{n}. #{item}" }.join("\n")
  end

  def inspect
    "#<TodoList #{size} items>"
  end
end

list = TodoList.new("wash", "shop")
puts list
p list

Output:

1. wash
2. shop
#<TodoList 2 items>

Enumerable provides count, but not size, empty?, << or []: those belong to the class, because only it knows how the elements are stored. Defining them makes the collection feel like an array to its callers:

def size
  @items.size
end

def empty?
  @items.empty?
end

def [](index)
  @items[index]
end

Challenge

Easy

Complete the class TodoList. << adds a task text and returns the list. each yields the tasks and returns to_enum(:each) without a block. Include Enumerable. size returns the number of tasks. to_s returns the tasks as numbered lines, 1. wash, or nothing to do, and inspect returns #<TodoList 3 items>.

The supplied code adds every input line by chaining <<, then prints the list, p of the list, the tasks with ! numbered through each.with_index(1) as 1! wash, the number of tasks containing o, and the first task alphabetically.

Your code goes in todo_list.rb. main.rb holds the supplied input/output code and cannot be edited.

Try it yourself

require_relative 'todo_list'

# Supplied input/output code: keep it as it is
input = []
while (line = gets)
  input << line.chomp
end
list = input.reduce(TodoList.new) { |l, task| l << task }
puts list
p list
list.each.with_index(1) { |task, n| puts "#{n}! #{task}" }
puts "with o: #{list.count { |t| t.include?("o") }}"
puts "first: #{list.min}"