Understanding Ruby - undefined local variable

Question

I've been playing around with Ruby and have come across this weird behavior. Could anyone please tell me why this is happening?

if I run:

def my_fn
  if false then
    a = {:a=>10, :b=>20}
  end
  puts a.class
  a || {}
end

and print the result the code compiles successfully and returns {}. But if I change the code to:

def my_fn
  puts a.class
  a || {}
end

it doesn't return {} but throws an Error. "'my_fn': undefined local variable or method 'a' for main:Object (NameError)"

Shouldn't a just be nil and not cause an error?

Community · Accepted Answer · 2017-05-23 12:05:31Z

9

This is because - The local variable is created when the parser encounters the assignment, not when the assignment occurs.

In the first code parser sees the line a = {:a=>10, :b=>20}, so a variable created, but no assignment happened. Thus a is nil. As per the all known facts a || {} returns {}.

In the second code there parser didn't see any assignment happened with a, so a has not been created as a local variable nor you create a method named as a. Thus when you tried to use a, got a valid error as you reported, which is undefined local variable or method 'a'.

edited May 23, 2017 at 12:05

CommunityBot

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answered Feb 11, 2014 at 14:17

Arup Rakshit

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2 Comments

chibi03 Over a year ago

I see, thanks. Do you know any advantages of using this approach? It doesn't seem very intuitive to me.

Arup Rakshit Over a year ago

@chibi03 I linked the documentation also, please go through that, plenty of knowledge there.

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