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states specifiable for its type), and its position (one of the site of the neigh-
borhood of the creating agent);
-
Clear(self )
: the agent deletes itself. This action has been introduced for sake
of garbage collecting agents that are no more operative. This fact is denoted
by a specific state (i.e. 'dead'). The agent that executes this action simply
clears itself and makes free its site.
4 MMASS Applications to MABS
The MMASS approach has been applied in the MABS domain to the localization
problem of suitable sites for extra-urban shopping centers. Simulations can be
useful when suitable space is available and a good location for a new shopping
center has to be chosen, or when the possibility of success in locating a new
shopping center in an area already served by other retailers has to be evaluated.
Finding a “good” location for a shopping center, that is, close enough to widely
populated areas and roads, and far away from other centers, is a problem that has
been widely studied by urbanists and economists, and many different approaches
have been proposed. In particular, the idea we implemented with agents is based
on the gravitational model, where shopping centers are seen as masses that
emit gravitational fields that attract consumers and repel other installations.
Geographical factors, such as the proximity of other centers and residential areas,
are essential for the choice of a suitable location for a shopping center. Moreover,
once some centers have settled in a given area, other factors like competition
should be taken into account. For example, one center could start low price
special sales in order to attract consumers, or invest heavily on advertisement.
This kind of model has been previously implemented with Cellular Automata [17,
5], but in the most significant examples the strict CA uniformity requirements
of CA formalism have been removed. In [15] it has been shown how the MAS
approach fits more naturally to this problem and domain, and how the design
of a MMASS composed by two MASS's allows the simulation of the two aspects
involved in this problem (Figure 1 shows an example of the system and the
user interface that has been developed. The area shown is the city of Bergamo
and its surroundings). Two MASS's have been defined: the territorial MASS
and the strategic MASS. In the former, the formation or the disappearance
of a shopping center is modelled by considering only geographical factors. In
the latter, already existing centers compete with one another trying to attract
consumers. Each shopping center is represented in both the territorial and the
strategic MASS. Interaction between the two MASS's is performed through field
emission by agents belonging to a MASS and their perception as external fields by
agents of the other MASS. Further investigation of the application of the MMASS
approach to socio-economic analysis in urban simulation will be continued within
a collaboration with the Austrian Research Center Seibersdorf (ARCS).
The MMASS model has been derived by the previously introduced Multi-
layered Reaction-Diffusion Machine (MRDM). In [18], it has been shown that a
special class of MRDM can simulate a deterministic Turing Machine and that, by
