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Ruby Ruby Booleans Build a Simple Todo List Program Part 3: Adding Todo Items

Roland Cedo
Roland Cedo
21,261 Points
Posted by Roland Cedo
Roland Cedo
21,261 Points

Pushing to todo_list_item array while it is an attr_reader?

It was to my understanding that attr_reader allows us to read an instance variable, but not write to it?

In this video, we create an instance variable called @todo_items and set it as an attr_reader:

class ToDoList
  attr_reader :name, :todo_items
  def initialize(name)
    @name = name
    @todo_items = Array.new()
  end
end

How are we able to execute the following code if we were only able to read todo_items?

def add_item(name)
  todo_items.push(TodoItem.new(name))
end

1 Answer

Kourosh Raeen
Kourosh Raeen
23,733 Points
Kourosh Raeen
Kourosh Raeen
23,733 Points

You don't need write access to push items into the todo_items array. You could even do that outside the class. Try adding this line:

todo_list.todo_items.push("Bread")

Here's the complete code:

require "./todo_item"

class TodoList
  attr_reader :name, :todo_items

  def initialize(name)
    @name = name
    @todo_items = []
  end

  def add_item(name)
    todo_items.push(TodoItem.new(name))
  end
end

todo_list = TodoList.new("Groceries")
todo_list.add_item("Milk")
todo_list.add_item("Eggs")

todo_list.todo_items.push("Bread")

puts todo_list.inspect

What wouldn't work is assigning something to todo_items, like a new empty array as in:

require "./todo_item"

class TodoList
  attr_reader :name, :todo_items

  def initialize(name)
    @name = name
    @todo_items = []
  end

  def add_item(name)
    todo_items.push(TodoItem.new(name))
  end
end

todo_list = TodoList.new("Groceries")
todo_list.add_item("Milk")
todo_list.add_item("Eggs")

todo_list.todo_items.push("Bread")

todo_list.todo_items = []

puts todo_list.inspect

You get an error saying : undefined method 'todo_items='

To fix that add an attr_writer:

require "./todo_item"

class TodoList
  attr_reader :name, :todo_items
  attr_writer :todo_items

  def initialize(name)
    @name = name
    @todo_items = []
  end

  def add_item(name)
    todo_items.push(TodoItem.new(name))
  end
end

todo_list = TodoList.new("Groceries")
todo_list.add_item("Milk")
todo_list.add_item("Eggs")

todo_list.todo_items.push("Bread")

todo_list.todo_items = []

puts todo_list.inspect

Now it works and you can see that the three items in the array are gone since you recreated an empty array.

Roland Cedo
Roland Cedo
21,261 Points
Roland Cedo
Roland Cedo
21,261 Points

Interesting, so calling a method to modify the the contents within the array (such as push, pop, shift, etc.) isn't considered "writing" to the instance variable? I suppose my misunderstanding came from thinking that it would be. It makes sense that variable assignment wouldn't be allowed in an attr_reader situation though.

Kourosh Raeen
Kourosh Raeen
23,733 Points
Kourosh Raeen
Kourosh Raeen
23,733 Points

That's correct. To be able to assign to an instance variable outside the class you need to either use attr_writer or define the setter method yourself.

Roland Cedo
Roland Cedo
21,261 Points
Roland Cedo
Roland Cedo
21,261 Points

That answers it, thanks for knowledge drop Kourosh!