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The use of the relational data model requires that the allocation of computer resources be automated. This means that
large and complex models can be constructed without the user having to worry about details such as array sizes and
scratch storage.
Structure of a GAMS Model
For the remainder of the tutorial, we will discuss the basic components of a GAMS model, with reference to the example
above. The basic components are listed in table Table 2.
Table 2: The basic components of a GAMS model
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.
通用代数建模系统(GAMS)是数学编程和优化的建模系统。它由一个语言编译器和一个稳定的集成各种高性能的求解器组成。GAMS适用于复杂的、大规模的建模应用,并允许您创建大的维护模型以很快的适应新的情况。
专注建模
GAMS允许用户在某种程度上,用跟数学描述非常相似的方式来制定数学模型。看一下这些例子就能说明GAMS模型的基本结构和特征以及与数学表达式的关系。GAMS让用户专注建模,通过要求简洁和的实体和关系规范,鼓励良好的建模习惯。GAMS语言与通用编程语言形式相似,因此对于有编程经验的人来说是熟悉的。由于模型的制定方式在某种程度上与它的数学描述类似,所以不仅是程序员,实际领域的也能理解和维护。GAMS专注于建模并且允许做所有相关的事。
陈述性知识和程序性要素的平衡混合,允许用户在GAMS中构建复杂的算法甚至实现分解方法。尤其是解决异常问题的模型,以及随之而来的性能问题。
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