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Model Library
When architects begin to design a new building, they develop the new structure by using ideas and techniques that have been
tested in previous structures. The same is true in other fields: design elements from previous projects serve as sources of
ideas for new developments.
From the early stages of the development of GAMS we have collected models to be used in a library of examples. Many of
these are standard textbook examples and can be used in classes on problem formulation or to illustrate points about GAMS.
Others are models that have been used in policy or sector analysis and are interesting for both the methods and the data they
use. All the substantive models in the library are described in the open literature. A collection of models is now included
with all GAMS systems, along with a database to help users locate examples that cover countries, sectors, or topics of interest
to them.
The syntax used to introduce features in the various chapters are presented using the Backus-Naur form (BNF) notation
where:
User Interface
Portability concerns also have implications for the user interface. The basic GAMS system is file-oriented, and no special
editor or graphical input and output routines exist. Rather than burden the user with having to learn yet another set of editing
commands, GAMS offers an open architecture in which each user can use his word processor or editor of choice. This basic
user interface facilitates the integration of GAMS with a variety of existing and future user environments.
Note that this simple example reveals some modeling practices that we regard as good habits in general and that are consistent
with the design of GAMS. First, all the entities of the model are identified (and grouped) by type. Second, the ordering
of entities is chosen so that no symbol is referred to before it is defined. Third, the units of all entities are specified, and,
fourth, the units are chosen to a scale such that the numerical values to be encountered by the optimizer have relatively small
absolute orders of magnitude. (The symbol $K here means thousands of dollars.)
The names of the types of entities may differ among modelers. For example, economists use the terms exogenous variable
and endogenous variable for given data and decision variable, respectively. In GAMS, the terminology adopted is as follows:
indices are called sets, given data are called parameters, decision variables are called variables, and constraints and the
objective function are called equations.
The GAMS representation of the transportation problem closely resembles the algebraic representation above. The most
important difference, however, is that the GAMS version can be read and processed by the computer.
Table 1: Data for the transportation problem (adapted from Dantzig, 1963) illustrates Shipping Distances from Plants to
Markets (1000 miles) as well as Market Demands and Plant Supplies.
选择使用的求解器非常简单---只要改变一行代码或者调整一个选项设置就可以了。想要比较求解器的性能或者看有什
么改进的可能,也不需要做任何的设置。同样的,模型类型可以轻松切换(比如:线性和非线性),尝试不同的公式也非
常的容易。通过使用GAMS,您可以得到一个广泛类型的模型和求解器的环境
科学软件网不定期举办各类公益培训和讲座,让您有更多机会免费学习和熟悉软件。