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If you submit a file containing the statements above as input to the GAMS program, the transportation model will be
formulated and solved. Details vary on how to invoke GAMS on different of computers, but the simplest ('no frills') way to
call GAMS is to enter the word GAMS followed by the input file's name. You will see a number of terse lines describing the
progress GAMS is making, including the name of the file onto which the output is being written. When GAMS has finished,
examine this file, and if all has gone well the optimal shipments will be displayed at the bottom as follows.
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c('Seattle','New-York') = 0.40;
is a valid GAMS assignment statement.
The same parameter can be assigned a value more than once. Each assignment statement takes effect immediately and
overrides any previous values. (In contrast, the same parameter may not be declared more than once. This is a GAMS error
check to keep you from accidentally using the same name for two different things.)
The right-hand side of an assignment statement can contain a great variety of mathematical expressions and built-in functions.
If you are familiar with a scientific programming language such as FORTRAN or C, you will have no trouble in becoming
14 A GAMS Tutorial by Richard E. Rosenthal
comfortable writing assignment statements in GAMS. (Notice, however, that GAMS has some efficiencies shared by neither
FORTRAN nor C. For example, we were able to assign c(i,j) values for all (i,j) pairs without constructing 'do loops'.)
The GAMS standard operations and supplied functions are given later. Here are some examples of valid assignments. In all
cases, assume the left-hand-side parameter has already been declared and the right-hand-side parameters have already been
assigned values in previous statements
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Note that set elements are stored as character strings, so the elements of t are not numbers.
Another convenient feature is the alias statement, which is used to give another name to a previously declared set. In the
following example:
Alias (t,tp);
the name tp is like a t0 in mathematical notation. It is useful in models that are concerned with the interactions of elements
within the same set.
The sets i, j, t, and m in the statements above are examples of static sets, i.e., they are assigned their members directly by the
user and do not change. GAMS has several capabilities for creating dynamic sets, which acquire their members through the
execution of set-theoretic and logical operations. Dynamic sets are discussed in Chapter Dynamic Sets. Another valuable
advanced feature is multidimensional sets, which are discussed in Section Multi-dimensional Sets.
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A scalar is regarded as a parameter that has no domain. It can be declared and assigned with a Scalar statement
containing a degenerate list of only one value, as in the following statement from the transportation model.
Scalar f freight in dollars per case per thousand miles /90/;
If a parameter's domain has two or more dimensions, it can still have its values entered by the list format. This is very useful
for entering arrays that are sparse (having few non-zeros) and super-sparse (having few distinct non-zeros).
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