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Greg Hauptmann

2006-Sep-02 03:55 UTC

When do I use "a" versus "@a" as a variable?

Hi,

Just realising I don''t quite understand the different between the 
variable name to use (say in a controller method) in terms of either
"a"
or "@a".  Both seem to work find re passing it to a local method in
the
class?

Tks

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Alex Wayne

2006-Sep-02 05:10 UTC

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Re: When do I use "a" versus "@a" as a variable?

Greg Hauptmann wrote:> Hi,
> 
> Just realising I don''t quite understand the different between the 
> variable name to use (say in a controller method) in terms of either
"a"
> or "@a".  Both seem to work find re passing it to a local method
in the
> class?
> 
> Tks
the @ symbol denotes an instance variable, without it they are local 
variables.  First I''ll show the difference in standard ruby:

  class Foo
    def initialize(value)
      @foo = value
      foo  = value
    end
    def instance_var
      @foo
    end
    def local_var
      foo
    end
  end

  var = Foo.new(''Hello World'')
  var.instance_var #=> ''Hello World''
  var.local_var    #=> ERROR: no method or local variable
''foo''

An instance variable retains it''s value for the life of the object and 
is exposed to all of the objects internal methods.  Local variables are 
destroyed as soon as the method they are defined in finishes execution.

In rails controllers however, all instance variables are passed to the 
views where the local variables are not:

  #controller action
  def show_var_scope
    @foo = ''Hello World''
    foo  = ''Goodbye World''
  end

  #app/views/show_var_scope.rhtml
  <%= @foo #yields ''Hello World'' %>
  <%= foo  #yields ERROR: undefined method or local variable %>

So use the instance variables in you controller action when you want the 
view to use the variable.  Otherwise just use local variables.

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Greg H

2006-Sep-02 08:07 UTC

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Re: When do I use "a" versus "@a" as a variable?

arrr - thanks heaps Alex


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Anil Wadghule

2006-Sep-03 16:10 UTC

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Re: When do I use "a" versus "@a" as a variable?

Thanks, Alex Wayne!

Regards,
Anil Wadghule

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Dominique Plante

2006-Sep-04 05:58 UTC

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Re: When do I use "a" versus "@a" as a variable?

Correct me if I''m wrong, but one thing to keep in mind about instance 
variables is that yes, it is true that they are available to the view as 
you showed so beautifully, but only on the current request...  That is, 
you must save to DB or session in order for another action to be able to 
get the value of @foo when a different view is displayed.

Is this correct?

Thanks!
Dominique

Alex Wayne wrote:> An instance variable retains it''s value for the life of the object
and
> is exposed to all of the objects internal methods.  Local variables are 
> destroyed as soon as the method they are defined in finishes execution.
> 
> In rails controllers however, all instance variables are passed to the 
> views where the local variables are not:
> 
>   #controller action
>   def show_var_scope
>     @foo = ''Hello World''
>     foo  = ''Goodbye World''
>   end
> 
>   #app/views/show_var_scope.rhtml
>   <%= @foo #yields ''Hello World'' %>
>   <%= foo  #yields ERROR: undefined method or local variable %>
> 
> So use the instance variables in you controller action when you want the 
> view to use the variable.  Otherwise just use local variables.

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Jatinder Singh

2006-Sep-04 13:03 UTC

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Re: When do I use "a" versus "@a" as a variable?

Thats right.
For every request a new instance of the controller is created and hence all
instance variables get the defaults!


Regards,
Jatinder


On 9/4/06, Dominique Plante
<rails-mailing-list-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>
wrote:>
>
> Correct me if I''m wrong, but one thing to keep in mind about
instance
> variables is that yes, it is true that they are available to the view as
> you showed so beautifully, but only on the current request...  That is,
> you must save to DB or session in order for another action to be able to
> get the value of @foo when a different view is displayed.
>
> Is this correct?
>
> Thanks!
> Dominique
>
> Alex Wayne wrote:
> > An instance variable retains it''s value for the life of the
object and
> > is exposed to all of the objects internal methods.  Local variables
are
> > destroyed as soon as the method they are defined in finishes
execution.
> >
> > In rails controllers however, all instance variables are passed to the
> > views where the local variables are not:
> >
> >   #controller action
> >   def show_var_scope
> >     @foo = ''Hello World''
> >     foo  = ''Goodbye World''
> >   end
> >
> >   #app/views/show_var_scope.rhtml
> >   <%= @foo #yields ''Hello World'' %>
> >   <%= foo  #yields ERROR: undefined method or local variable %>
> >
> > So use the instance variables in you controller action when you want
the
> > view to use the variable.  Otherwise just use local variables.
>
>
> --
> Posted via http://www.ruby-forum.com/.
>
> >
>

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Aryk Grosz

2007-Apr-11 05:28 UTC

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Re: When do I use "a" versus "@a" as a variable?

also for example if you want to do something like this

def test
   if g>h
      @checked = true
      <more stuff>
   end
   <some more stuff>
   new_var = 6 if @checked
end


if you used just normal "checked" you would have to use defined?
checked
in the if condition since its a local variable.

You never have to worry with "@a" variables because it will default to
nil and not throw an error.

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Rails - Sep 2006 - When do I use "a" versus "@a" as a variable?

When do I use "a" versus "@a" as a variable?

Re: When do I use "a" versus "@a" as a variable?

Re: When do I use "a" versus "@a" as a variable?

Re: When do I use "a" versus "@a" as a variable?

Re: When do I use "a" versus "@a" as a variable?

Re: When do I use "a" versus "@a" as a variable?

Re: When do I use "a" versus "@a" as a variable?