adding methods to instance objects

Question

Hi guys I have a question on adding methods to instance objects. There is something I don't get.

I'm trying to do a binary search tree in Ruby. So I made a node class like such:

class Node
    attr_accessor :value, :right, :left

    def initialize(value)
        @value = value  
    end
end

So when I want to create an instance object from that class I do this:

tree = Node.new(10)

But if I want to create a binary search tree I need left and right pointers for values that are less and higher than my root number.

So that should go something like this.

tree.left = Node.new(8)
tree.right = Node.new(13)

And if I want to go further I do this:

tree.left.left = Node.new(7)

And from my inspect method that I've adapted i get this:

"{10::{8::{6::nil|nil}|nil}|nil}"

But from this:

left=Node.new(1).left = Node.new(2).left = Node.new(3)

I get:

{3::nil|nil}

So, why doesn't this method chaining work like on the previous example?

Thanks

Paweł Dawczak · Accepted Answer · 2015-04-24 10:13:57Z

1

tap will help in your case:

left = Node.new(1).tap do |n|
  n.left = Node.new(2).tap do |n|
    n.left = Node.new(3)
  end
end

Nice article about how you can use the tap method.

Give it a shot, and let know if it worked!

Good luck!

edited Apr 24, 2015 at 10:13

answered Apr 24, 2015 at 10:01

Paweł Dawczak

9,6492 gold badges27 silver badges38 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Piotr Jakubowski · Accepted Answer · 2015-04-24 13:00:31Z

1

In ruby the value of assigning something to a variable is the value you are assigning. Which means that if you do

a = 5

The value of that expression is 5. Moreover, assignments are done right to left. Which means that in your code:

left=Node.new(1).left = Node.new(2).left = Node.new(3)

You first create Node.new(3) and then create Node.new(2) and assign the Node.new(3) to left of the 2 node. The result of this assignment is Node 3 which you assign to left of Node 1. Again the result of this assignment is Node 3 which you again assign to your variable left. So in the end it acts as if you just assigned Node 3 to left, but doing 2 unnecessary object instantiations on the way.

answered Apr 24, 2015 at 13:00

Piotr Jakubowski

1,75010 silver badges16 bronze badges

3 Comments

Tomislav Mikulin Over a year ago

tnx for the answer, but how does this work then: tree.left.left = Node.new(7) //// "{10::{8::{6::nil|nil}|nil}|nil}"

Piotr Jakubowski Over a year ago

In that case you first set tree = Node10, then tree.left = Node8. Then if you do tree.left.left = Node.new(7) is the same as if you did a = tree.left # => Node 8 a.left = Node.new(7) # Which means you set Node 7 to left of Node 8 This is different than left=Node.new(1).left = Node.new(2).left = Node.new(3)

Tomislav Mikulin Over a year ago

tnx for you answer Piotr, so your saying that this would work left=Node.new(1).left = Node.new(2).left = Node.new(3) - If i did it on an instance object like tree = Node.new(10) like the following, tree.left=Node.new(1).left = Node.new(2).left = Node.new(3) - But how is it that I can append the same instance method .left with different new objects with different values in a row but I can't do it without an instance object like tree? Tnx Tom

Collectives™ on Stack Overflow

adding methods to instance objects

2 Answers 2

Comments

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related