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In what circumstances should instance variables be used in place of other types of variables?

I am using Ruby on Rails 3 and I would like to know under what circumstances should I use instance variables instead of other types of variables and if there is a security reason for them.

Example:

# Using an instance variable
@accounts = Account.find(...)

# Using a "local"\"normal" variable
account = Account.find(...)
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2 answers

In the general case, an instance variable is local and stored inside an object instance, while a local variable is only local and stored inside a function / object / block area. For instance:

class User
  def name
    @name
  end

  def name= name
    @name = name
  end
end

def greet user
  name = user.name || 'John'
  p "Hi, my name is #{name}"
end

user = User.new
greet user
=> 'Hi, my name is John'
name
=> NameError: undefined local variable or method 'name' for main:Object
user.name = "Mike"
greet user
=> 'Hi, my name is Mike'
@name
=> nil

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