Personal Computer Example
In the following example based on the personal computer all files and folders are
attributes. In addition files, but not folders, have a
Model of a Personal Computer.
In any given situation, getting to the right blend of entity types, attributes and
may be an iterative process as demonstrated in this example.
For this example we start with two entries for chemical elements from a scientific
oxygen (Chem.). A nonmetalic element, symbol O, at. no. 8, r.a.m. 15.994, valency
2. It is a colourless, odourless gas which supports combustion and is essential for
the respiration of most forms of life. M.p. -218℃, b.p.-183℃, density 1.42904 g/dm3 at s.t.p., formula O2. An unstable form is ozone, O3. Oxygen is the most abundant element, etc.
chlorine (Chem.). Element, symbol Cl, at.no. 17, r.a.m. 35.453, valencies 1-,3+, 5+,
7+, m.p. -101℃, b.p. -34.6℃. The second halogen, chlorine is a geenish yellow gas,
with an irritating smell etc.
From these entries, and with some expansion of abbreviations, I surmise that each
element has a name, a symbol, an atomic number, a relative atomic mass, one or more
valencies, a melting point, a boiling point, and, optionally maybe, a density. I am
left unsure whether all elements have a formulae, or whether they may have forms which
may have formulae which is probably closer to the truth. I provisionally model this
situation on an entity model diagram like so:
I found this preliminary model to be inadequate once I read the entry for sulphur:
sulphur (Chem.). A nonmetalic element occurring in several allotropic forms. Symbol
S, at. no. 16, r.a.m. 32.06, valencies 2,4,6. Rhombic(α-) sulphur is a lemon yellow
powder; m.p. 112.8 ℃, rel. d. 2.07. Monoclinic (β-) sulphur has a deeper colour than
the rhombic form; m.p. 119 ℃, rel.d. 1.96, b.p. 444.6C. Chemically, sulphur resembles
Reading this third entry, I learn that different forms of sulphur have different melting
and that my provisional model has miss-positioned the 'melting point' attribute
in that chemical elements, in and by themsleves, cannot be attributed melting points.
After a little further research I learn that these forms taken by chemical elements
are, technically speaking, allotropes
and that it is these that have melting-points, boiling points and densities. I reach
In tabular form the dictionary entries for oxygen, chlorine and sulphur can be structured
like this2 :
In this table:
the columns correspond to attributes - each column heading is the name of an attribute,
- the rows correspond to subject entities,
Each cell presents the value
of an attribute of a subject entity.
- Some rows have other rows inside them representing the parts of subject
entities i.e the compositional substructure.
The overall structure of each row, considered as a messages, we will represent
by these message structure definitions: