SIMSCRIPT II.5 Programming Language

Modelling Concepts
every COMMUNITY owns some MASONS
every MAN may belong to the MASONS
The entity structures shown contain the automatically-generated ownership and membership pointers
F.MASONS, L.MASONS, N.MASONS, P.MASONS, S.MASONS, and M.MASONS.
COMMUNITY
F.MASONS
L.MASONS
N.MASONS
MAN1
P.MASONS
S.MASONS
M.MASONS
MAN 2
P.MASONS
S.MASONS
M.MASONS
MAN 3
P.MASONS
S.MASONS
M.MASONS
MAN 4
P.MASONS
S.MASONS
M.MASONS
Figure 4-5. Owner-member Set Relationships
The set owner, the entity named COMMUNITY, has two attributes that point to the member entities
that are logically first and last in the set MASONS. It also has an N.MASONS attribute, which in this
case is 4. The member entities, here called MAN1, MAN2, MAN3, and MAN4, have two attributes
that point to the members of the set that logically precede and succeed them. Thus, F.MASONS in
COMMUNITY points to the entity structure of MAN1, indicating that it is the first entity (logically) in
the set MASONS. The pointer P.MASONS of MAN1, points nowhere (has a zero value), as MAN1 has
no predecessor in MASONS. Its S.MASONS pointer, however, points to MAN2, which logically
follows it in MASONS. As shown, P.MASONS of MAN2 points back to its predecessor, MAN1. The
same is true of MAN3. MAN4, as the last member of MASONS, differs somewhat. It has no successor
(S.MASON = 0), and is pointed to directly by L.MASONS, the last-in-set pointer of COMMUNITY.
Each MAN also has an M.MASONS attribute which is non-zero since each MAN is a member of the
set MASONS.
The items to note from this example are:
1. A set is made up of entities that point to one another, thereby expressing their member relationships.
2. First-in-set and last-in-set pointers connect a set's owner to its member entities.
3. A specific entity can own or belong to any number of sets as long as it has the required
pointer attributes. For example, the entity MAN might own the set CHILDREN whose members
are also entities of the type MAN. These relationships might be defined by the statement:
every MAN may belong to the MASONS,
own some CHILDREN, and belong to the CHILDREN
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