From: Brian Candler <b.candler@...>
Date: 2008-10-21T16:59:51+09:00
Subject: Re: Can anyone tell me why my code isn't working?

Adam Penny wrote:
> For some reason the get_mac_by_printer(host_name) method isn't working.
> All the other bits work, but I have a feeling that I'm running into a
> problem with variables defined within blocks, but I'm at a loss as to
> how to negotiate it.

No, nothing to do with variable scoping.

> The resulting mac is returning as nilClass and it
> definitely isn't nil! Code as follows:
...
>   def get_mac_by_host(host_name)
>     servers=@plist_hash['servers'].each do |s|
>       mac=s['mac'] if s['name']==host_name
>       return mac
>     end
>   end

It definitely *is* nil.

Note that your 'each' loop will onlyever run for one iteration, since 
'return mac' will always return from the enclosing method 
(get_mac_by_host) during the first iteration. So, unless the passed 
host_name happens to match the first entry of @plist_hash['servers'], 
then the assignment

   mac = s['mac'] if s['name']==host_name

will not be executed (because it's qualified by "if ... false condition 
..."), so the value of the local variable 'mac' will be nil.

To demonstrate:

  if 1 == 2
    foo = "oops"
  end
  puts foo     # nil

So your loop is similar to:

  def mycode
    (1..5).each do |v|
       res = v if v == 3
       return res
    end
  end
  puts mycode   # nil

You could fix this by changin "return res" to "return res if v == 3", or 
"return res if res", but more simply

  def mycode
    (1..5).each do |v|
      return v if v == 3
    end
  end
  puts mycode   # 3
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