Quick start - Gams

esourceusage(resources,products) *production(products))
=L= endowments(resources) + HireResource(Resources);
Hirelimit(Resources)..
HireResource(Resources) =l= hiredata(resources,"maxavailable");
MODEL RESALLOC /ALL/;
SOLVE RESALLOC USING LP MAXIMIZING PROFIT;
where only the material in black was added with no alterations of that in red relative to the
newcontext.gms example. So what? The algebraic structure from the other study could be used
supplied the core of the new model with structural features added as needed. Such a capability
constitutes another major GAMS model development strategy.
One can adapt models from other studies customizing them for the problem at hand speeding up
the development process. In addition to adapting models from related studies done by the
modeler or the group in which the modeler works, there are number of other sources one may be
able to exploit to jumpstart a model development project. This is further discussed below.
Aid with initial formulation and subsequent changes
GAMS aids both in initially formulating and subsequently revising formulations. GAMS
facilitates specification and debugging of an initial formulation by allowing the modeler to begin
with a small data set, then after verifying correctness expand to a much broader context. For
example, one could initially specify a small transportation model with a few suppliers and
demanders. Then after that model is debugged one could expand the problem to encompass fifty
shipping origins and two hundred destinations without needing to change the algebraic model as
discussed in the Small to Large: Aide in Development and Debugging chapter and the
expandability section above.
GAMS also makes it easy to alter the model. Large models in programs like spreadsheets can be
difficult to modify. In a spreadsheet, I find it hard to add in a set of new constraints and
variables properly interjecting all the linkages and cannot figure out how to easily get a model
right with a few commodities then automatically expand the model scope to many commodities
and locations as illustrated in the expandability section above. On the other hand, GAMS allows
one to add model features much more simply. Generally, modelers do not try to make a
complete formulation the first time around. Rather one starts with a small formulation and then
adds structural features as needed adding features as illustrated in the augmentation section
above. GAMS also enforces consistent modeling, allowing models to be transferred between
problem contexts as shown above.
Adding report writing
Generally, default GAMS output for the model solution is not adequate for conveying solution
information to the modeler or associated decision-makers. One often does calculations using
solution information to improve information content of the GAMS output. This is elaborated
upon in the Report Writing chapter.
Self-documenting nature
Courtesy of B.A. McCarl, October 2002 44