Information Technology Reference
In-Depth Information
It is worth emphasizing how flexible and dynamic the linking of sub-ETs
became with this new method, allowing not only the use of any kind of link-
ing function (even functions of just one argument can now be used as linkers)
but also the use of different linking functions at a time as opposed to just one
static linker. And to top it off, this new system is totally unsupervised, being
the linking functions totally sorted out by the evolutionary process itself.
As Figure 2.7 clearly shows, this kind of representation not only allows
the evolution of linking functions but also allows code reuse. Thus, this is
also an extremely elegant form of implementing ADFs in gene expression
programming. Indeed, any ADF in this cellular representation can not only
be used as many times as necessary but also establish different interactions
with the other ADFs in the main program or cell. For instance, in the particu-
lar case of Figure 2.7, ADF
0
is used twice in the main program, whereas
ADF
1
and ADF
2
are both used just once.
It is worth pointing out that homeotic genes have their specific length and
their specific set of functions. And these functions can take any number of
arguments (functions with one, two, three, ...,
n
arguments). For instance, in
the particular case of chromosome (2.17), the head length of the homeotic
gene
h
H
is equal to five, whereas for the conventional genes
h
= 4; and the
function set of the homeotic gene consists of F
H
= {+, *, /, Q}, whereas for
the conventional genes the function set consists of F = {+, -, *, /}.
In summary, as Figure 2.7 emphasizes, the cellular system of gene expres-
sion programming is not only a form of elegantly allowing the totally uncon-
strained evolution of linking functions in multigenic systems, but also an
extremely elegant and flexible way of encoding automatically defined func-
tions that can be called an arbitrary number of times from an arbitrary number
of different places.
2.3.2 Multicellular Systems with Multiple Main Programs
The use of more than one homeotic gene results obviously in a multicellular
system where each homeotic gene puts together a different combination of
sub-expression trees.
Consider, for instance, the following chromosome:
012345601012345601012345601
01234560123456
/Q+*babab/+a/abbab*Q-bbaaab
*1+1020*Q*1202
(2.18)
It codes for three conventional genes and two homeotic genes (shown in
