As a beginner, I've started to
learn GAMS doing the examples published in “McCarl GAMS User Guide”. The first
model shown in the guide is a simple optimization problem like this:
SET j
/maiz,trigo,algodon/
i
/tierra,trabajo/;
PARAMETER
c(j)
/maiz 109, trigo 90, algodon 115/
b(i)
/tierra 100, trabajo 500/;
TABLE a(i,j)
*table must be aligned
maiz
trigo algodon
tierra 1 1 1
trabajo 6
4 8 ;
Placing the
numbers on a table in the same code can be useful in some cases but in general
we build the tables in programs like EXCEL:
If we want to import this table to
GAMS, EXCEL must be installed in our system and the .xls file should be saved in
“projdir” folder (in this case results.xls). Then we can use the GDXXRW tool. This
tool converts .xls to .gdx files which can be read by GAMS. Writing the code:
$CALL
GDXXRW.EXE results.xls par=data rng=sheet1!A1:D3
parameter
data(i,j);
$gdxin
results.gdx
$load
data
display
data
The first line allows to read
results.xls and generates a .gdx file. par=data defines that the table is a
parameter named "data". Finally we specify the range and the sheet
that we want to read.
The second line defines the set of the
parameter data.
The third line loads the .gdx file.
The last line loads the parameter.
And the entire code:
SET j
/maiz,trigo,algodon/
i
/tierra,trabajo/;
PARAMETER
c(j)
/maiz 109, trigo 90, algodon 115/
b(i)
/tierra 100, trabajo 500/;
$CALL GDXXRW.EXE
results.xls par=data rng=sheet1!A1:D3
parameter data(i,j);
$gdxin results.gdx
$load data
display data
POSITIVE VARIABLES x(j);
VARIABLES BENEFICIO ;
EQUATIONS objetivo ,
restriccion(i) ;
objetivo.. BENEFICIO =e= SUM(J,(c(J))*x(J)) ;
restriccion(i).. SUM (J,data(i,J)*x(J)) =L= b(i);
MODEL RESALLOC /ALL/;
SOLVE RESALLOC USING LP
MAXIMIZING BENEFICIO;
References:
McCarl, B., et al. (2011). McCarl GAMS User Guide. GAMS Development Corporation.
