In my actions, when should I use @variable_name, only when I need it in a view?

Do I only prefix var names with @ i开发者_StackOverflow社区f I need that variable in a view?

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Basically you want to use @variable_name (instance variables) when you want to access these variables outside of the local scope, one example is using them in the view.

Another example:

before_filter :setup

def index
# can access @profile
end

private

def setup
  @profile = Profile.find(params[:profile_id])
end