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  • Tabulka s obsahem
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  • Hodnocené. / 5. Na základě hodnocení zákazníků
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Introduction
1-11
Town = 5
Street = 6
HouseNo = 7
Bedrooms = 8
Bathrooms = 9
TotalSquareFeet = 10
HowMuchILikeIt = 11
You set up attributes to describe carsattributes such as list price, year of
manufacture, make, model, automatic or manual transmission, color, number of
people it will fit, dealer’s name, reputation on a scale of 1 to 10, and how much
you like it on a scale of 1 to 10. You assign each attribute a unique number called
an attribute identifier. For example, you use 1 for list price, 2 for year of
manufacture, 3 for make, etc. To access the attribute reputation, you can specify
the object by name (“Car”) or by identifier (“2”), and the attribute by name
(“Reputation”) or number (“8”) in the format “Car.Reputation” or “2.8.
Whenever the number of values of an object is limited, you assign code numbers
to the values. For example, for the Color attribute of the object House, you use
1 for white, 2 for green, 3 for brown, etc.
Until now, we’ve been discussing houses and cars in abstractions, not physical
manifestations. For example, houses have lot sizes and colors, but a particular
house you have seen has a 10,000 square foot lot and is blue. To reference
particular houses and cars, you select attributes that, by themselves, uniquely
identify them. The town, street, and house number uniquely identify a house, but
the town alone or the color does not. The dealer, year, make, and model uniquely
identify a car, but the list price alone does not. So you select the format
“House.Street.Town” to uniquely identify houses, and the format
“Dealer.Year.Make.Model” to uniquely identify cars. The MIB name for a
collection of attribute values that you use to identify the physical manifestation of
an object is an instance. Thus, House.Street.Town is the instance format of the
object House, and 221.Main.Middleton is an instance of that object.
Table 1-
2 shows the object House, its attributes, its instances, and its values. The
attributes and instances are in bold, and the values are in regular print.
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