list.rb

class List
  attr_reader :title, :tasks

  def initialize(title, tasks = [])
    @title = title
    @tasks = tasks
  end

  def add_task(task)
    tasks << task
  end

  def complete_task(index)
    tasks[index].complete!
  end

  def delete_task(index)
    tasks.delete_at(index)
  end

  def completed_tasks
    tasks.select { |task| task.complete? }
  end

  def incomplete_tasks
    tasks.select { |task| !task.complete? }
  end
end

list_spec.rb

require "rspec"

require_relative "list"

describe List do
  # Your specs here
end

task.rb

class Task
  attr_reader :description

  def initialize(description)
    @description = description
    @complete = false
  end

  def complete?
    @complete
  end

  def complete!
    @complete = true
  end
end

task_spec.rb

require "rspec"

require_relative "task"

describe Task do

  let(:description) { "Walk the giraffe" }
  let(:task)        { Task.new(description) }

  describe "#initialize" do
    it "takes a description for its first argument" do
      Task.new("Feed the parakeet").should be_an_instance_of Task
    end

    it "requires one argument" do
      expect { Task.new }.to raise_error(ArgumentError)
    end
  end

  describe "#description" do
    it "returns the correct description for the task" do
      task.description.should eq description
    end
  end

  describe "#complete?" do
    it "returns false for incomplete tasks" do
      task.complete?.should be_false
    end

    it "returns true for completed tasks" do
      task.complete!
      task.complete?.should be_true
    end
  end

  describe "#complete!" do
    it "changes the task from incomplete to completed" do
      #
      # Note the use of the `be_complete` method here.
      # This is some RSpec goodness which expects a
      # method `complete?` to be defined which returns
      # true or false.
      #
      task.should_not be_complete
      task.complete!
      task.should be_complete
    end
  end
end