How to display the attribute of an object as a string in Ruby

Question

I have two classes List and Task:

class List
  attr_reader :all_tasks

  def initialize
    @all_tasks = []
  end

  def add (task)
    @all_tasks << task
  end

  def show
    all_tasks
  end
end

class Task
  attr_reader :description

  def initialize (description)
    @description = description
  end
end

And the following code:

breakfast = Task.new("Make Breakfast")

my_list = List.new
my_list.add(breakfast)
my_list.add(Task.new("Wash the dishes!"))
my_list.add("Send Birthday Gift to Mom")

puts "Your task list:"
puts my_list.show

Output:

Your task list:
#<Task:0x00007fd9e4849ed0>
#<Task:0x00007fd9e4849e30>
Send Birthday Gift to Mom

I want to be able to show the tasks of the to-do list as a string and in the same time have the Task instances as objects inside the array. How do I do that?

Note that although attr_reader :all_tasks looks like read-only, it allows the caller to modify all_tasks, e.g. my_list.all_tasks << 'another task'. — Stefan
– Stefan, Commented Oct 17, 2017 at 7:17

Stefan · Accepted Answer · 2017-10-17 07:29:38Z

3

Considering the code in your question, it would suffice to simply redefine method to_s of Task.

class Task
    attr_reader :description
    def initialize (description)
      @description = description
    end

    def to_s
      "Task: #{description}"
    end
end

Output

Your task list:
Task: Make Breakfast
Task: Wash the dishes!
Send Birthday Gift to Mom

edited Oct 17, 2017 at 7:29

Stefan

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answered Oct 17, 2017 at 7:05

Sergio Tulentsev

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4 Comments

Stefan Over a year ago

It might be worth noting that this works because puts prints each element on a new line if called with an array. To have more control over the output, List#show should return a string.

Sebastian Peter Over a year ago

Thank you that really helped. Since I am only started to learn ruby(my second week), i would like to now, why the moment I write the def to_s, ruby doesn't display the string representing the output anymore?

Sebastian Peter Over a year ago

@Stefan Can you please also explain what List#show is/means and how to implement it? It might be stupid question but I won't learn anything if i don't ask :-) Thank You again for your help

Stefan Over a year ago

@SebastianPeter List#show refers to the show instance method within your List class. It currently returns an array, whereas it probably should return a string. I've posted an answer showing a slightly different approach.

Stefan · Accepted Answer · 2017-10-17 08:19:08Z

1

You call add with Task instances:

my_list.add(Task.new("Wash the dishes!"))

and with String instances:

my_list.add("Send Birthday Gift to Mom")

Having a mixture of both, Task and String instance in one array makes it harder to work with. Unless you really want or need this, I would change add, so it converts string arguments to Task instances:

class List
  # ...

  def add(task)
    task = Task.new(task) unless task.is_a? Task
    @all_tasks << task
  end
end

is_a? checks whether task is (already) a Task. If not, it is passed as an argument to Task.new which returns such instance. This ensures, that @all_tasks only contains Task instances.

Your current implementation of List#show simply returns all_tasks, i.e. an array. Although puts is able to print arrays ...

If called with an array argument, writes each element on a new line.

... I would change show to return a formatted string:

class List
  # ...

  def show
    all_tasks.map { |task| "[ ] #{task.description}" }.join("\n")
  end
end

map returns a new array with a string for each task instance. Each string contains the corresponding task's description, prefixed by [ ] which should resemble a little check box. join then concatenates these string elements using "\n" (newline) as a separator.

Output:

Your task list:
[ ] Make Breakfast
[ ] Wash the dishes!
[ ] Send Birthday Gift to Mom

edited Oct 17, 2017 at 8:19

answered Oct 17, 2017 at 7:47

Stefan

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1 Comment

Sebastian Peter Over a year ago

Thank You again. This is even better. I feel like i made a big jump in understanding Ruby.

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