OO question

[Dan Sommers]

On 1 Jan 2007 17:09:19 -0800,
fejkadress at hushmail.com wrote:

> Lets say I have those two classes, Person and Address. How would one
> implement the relationship between them? First, a Person can have one
> or more addresses (or none), that could be represented as a list of
> Addresses, right? But then, if I have an Address I want to see which
> persons who live there, then I would have a list of Persons in each
> Address.

> Is this doubly-linked way of doing it a good way of doing it, or is
> there a better "OO way" I haven't learned yet?

I don't know about "a better OO" way, but one way is to model the
various relationships as a list of (person, address) pairs:

    persons = [ Person( ), Person( ), Person( ) ]
    addresses = [ Address( ), Address( ), Address( ), Address( ) ]

    livesat = ((person[ 1 ], address[ 1 ]),
               (person[ 2 ], address[ 1 ]),
               (person[ 2 ], address[ 2 ]))

    worksat = ((person[ 1 ], address[ 3 ]),
               (person[ 2 ], address[ 4 ]))

Then the list of people who live at address[ 1 ] is:

    [ (person, address) for (p, a) in livesat if a is address[ 1 ] ]

But what about all the various relationships (lives at, works at, is
known to have vandalized, etc.)?

    livesat = Relationship( "lives at" )
    worksat = Relationship( "works at" )
    vandalized = Relationship( "vandalized" )

    information = ((person[ 1 ], address[ 1 ], livesat),
                   (person[ 2 ], address[ 1 ], livesat),
                   (person[ 2 ], address[ 2 ], livesat),
                   (person[ 1 ], address[ 3 ], worksat),
                   (person[ 2 ], address[ 4 ], worksat),
                   (person[ 1 ], address[ 4 ], vandalized))

Now the list of people who live at address[ 1 ] is:

    [ (person, address) for (p, a, r) in information
      if a is address[ 1 ] and r is livesat ]

(There are other ways, too, such as capturing one person and one address
right inside a Relationship; someone with a more comp-sci-theoretical
background than I have can probably tell you why or when one way might
be better than the other.)

The definitions of Person, Address, and Relationship, as well as the
user-interface(s) and the persistent storage mechanism(s), are left as
exercises to the interested reader.  ;-)

Regards,
Dan

-- 
Dan Sommers
<http://www.tombstonezero.net/dan/>
"I wish people would die in alphabetical order." -- My wife, the genealogist