Ah I see... there are three ways to go about this.

1. Set `@postal_codes` in the constructor. Any methods you define can then access `@postal_codes`.
2. Set `postal_codes` (without an `@`) as the first line of the class. You should then be able to access `postal_codes` in any method you define. This has the added benefit of hiding the variable from any users of the class.
3. Set `@postal_codes` (with an `@`) as the first line of the class.

What do you think?