From: Mischa Fierer <f.mischa@...>
Date: 2008-08-28T18:54:23+09:00
Subject: Confused by scope in ruby

Hello,

I've been writing in ruby for a bit, but ran into something while
working on a project that struck me as weird, and could not find a ready
explanation.

Sorry if this is something super basic...

Here is a simplified example.

My question is: Why is the hash not empty at the end?

class Hmmm
  def why_not_empty?
    hash = {}
    puts "Start: hash = #{hash.to_s}"
    [1, 2, 3, 4, 5].each do |num|
      put_in_hash(num, hash)
    end
    puts "End hash:\n#{hash.to_s}" # i would expect this to be {}
  end

  def put_in_hash(num, hsh)
    hsh[num.to_s] = "hello from hash index #{num}.\n"
    return nil
  end

  def number
    num = 2
    puts "Start number: #{num}"
    change_number?(num)
    puts "End number: #{num}"
  end


  def change_number?(num)
    num = 3
  end
end

@a = Hmmm.new
@a.why_not_empty?
@a.number



Output:
Start: hash =
End hash:
1hello from hash index 1.
2hello from hash index 2.
3hello from hash index 3.
4hello from hash index 4.
5hello from hash index 5.
Start number: 2
End number: 2

The number, as I would expect, stays at two, but the hash is actually
changed by the method, as if I were passing it a pointer in C or
something...
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