From: Calamitas <calamitates@...>
Date: 2008-07-01T19:12:01+09:00
Subject: Re: Dynamic Variables

On Tue, Jul 1, 2008 at 11:52 AM, Igal Koshevoy <igal@pragmaticraft.com> wrote:
> Marc Heiler wrote:
>>
>> Is there any way in ruby to create dynamic variables?
>>
>> counter = 52
>> box#{counter} = "cat and dogs"
>>
>> There would be then a new variable
>> box52
>>
>> Note that this is not a question whether someone should do it or not -
>> this is solely whether it is doable or not. Until today I thought it is
>> somehow possible with eval but I failed, so I assume it is not possible.
>>
>
> irb(main):010:0> counter = 52
> => 52
> irb(main):011:0> eval "box#{counter} = 'cat and dogs'"
> => "cat and dogs"
> irb(main):012:0> box52
> => "cat and dogs"

Now try the same thing in plain ruby without irb. Never trust irb on
things like these.

Dynamic variables as created by eval live in a separate, eval-specific
scope and can only be accessed through eval. This is a consequence of
Ruby deciding at "compile" time what's a local variable and what's
not. So, if you have code like this:

   eval "a = 5"
   p a

then in "p a", a is determined to be a method call before that code is
even run. The a is accessible through eval though:

  eval "a = 5"
  p eval("a")

Peter