Power of GAMS
AGEC 641 Lab, Fall 2011
Mario Andres Fernandez
Based on material written by Gillig and McCarl. Improved upon by many previous
lab instructors.
Special thanks to Yuquan “Wolfgang” Zhang
Power of GAMS
• Algebraic Modeling
 Context Changes
 Expandability of Models
 Adding Conditionals
• Adding Report Writing/Self Documenting Nature
• Small to Large Model Development
• Model Library
• Use by Others
SETS
Process
Production process
/Corn
Corn production
Wheat
Wheat production
Cotton Cotton production
/
Resource
Resource item used
/Land
Land used by process
Labor Labor used by process/;
SETS
Process
/MakeChair
MakeTable
MakeLamp
Resource
/PlantCap
Labor
Production process
Chair production
Table production
Lamp production
/
Resource item used
Production Capacity
Labor used by process/;
Context Changes
The same algebraic
model can be used in
multiple contexts
often with the same or
very similar structure.
TABLE ResourceUse(Resource,Process) Resource
used
Corn Wheat Cotton
Land
1
1
1
Labor
6
4
8
;
PARAMETERS
Revenue(Process)
Revenues from process
/ Corn 109,
Wheat 90,
Cotton
115 /
ResourceAvailable(Resource) Resource
availability
/ Land
100,
Labor
500 / ;
TABLE ResourceUse(Resource,Process) Resource
used
MakeChair MakeTable
MakeLamp
PlantCap
2
3
1.1
Labor
1
2
0.5 ;
PARAMETERS
Revenue(Process) Revenues from process
/ MakeChair 11, MakeTable 10, MakeLamp 12/
ResourceAvailable(Resource) Resource
availability
/ PlantCap
12,
Labor
5 / ;
Context
Changes
One has to
revise data
and their set
elements for
consistency;
yet, other
structures
remain
Crop production problem
Expand the Scope
Instead of growing 3 crops, now a farmer also wants to grow
soybeans. One needs only to modify an element in PROCESS set,
resource usage data, and revenues.
SETS
Process
/Corn
Wheat
Cotton
Soybeans
Resource
/Land
Labor
Production process
Corn production
Wheat production
Cotton production
Soybeans production
/
Resource item used
Land used by process
Labor used by process/ ;
Expand the Scope (cont)
Other data and the model structure remains the same
TABLE ResourceUse(Resource,Process)
Resource used
Corn
Wheat Cotton Soybeans
Land
1
1
1
1
Labor 6
4
8
4 ;
PARAMETERS
Revenue(Process) Revenues from process production
/ Corn
109
Wheat
90
Cotton
115
Soybeans
95
/
ResourceAvailable(Resource) Resource availability
/ Land
100
Labor
500 / ;
Augmenting Existing Models
If wheat is used for farmer consumption, it must be grown in at least
10 units (acres). Modification includes adding data on minimum land
use, and equation specification on minimum land use.
PARAMETERS
Revenue(Process) Revenues from process production
/ Corn
109
Wheat
90
Cotton
115
Soybeans
95
/
ResourceAvailable(Resource) Resource availability
/ Land
100
Labor
500 /
MinLand(Process) Minimum land requirement
/ Corn
0
Wheat
10
Cotton
0
Soybeans
0 / ;
Augmenting Existing Models (cont)
Other data and the model structure remains the same
EQUATIONS
Objective
ResourceEq(Resource)
MinLandReq(Process)
Maximize farm income
Resource constraint
Minimum land requirement;
MinLandReq(Process)..
ResourceUse(“Land”, Process)*Production(Process)
=G= MinLand(Process);
Self-Documenting Nature
• Can you figure out what context the example below is from?
• Can you figure out what context the example below is from?
Self-Documenting Nature
One can include comments in the code by
• Using * in the first character position. The remaining
characters in the line to be ignored but printed on the output
file.
• Setting off by $ONTEXT and $OFFTEXT.
Small to Large Modeling
• GAMS expandability allows the same model structure,
calculations, and report writing to be used with SETS with few
elements vs. SETS with many items.
• Using a small data set allows up to examine the model
structure and function easier and better.
• To expand, one uses SmallStocks(Stocks) = YES;
Model Library
Modelers can
find a number of
sources of
models from
which they can
expand or take
insights.
Use By Others
• Create a “t” folder under the project directory
• When finished running data.gms, GAMS will
save all the information in \t\a1
• To solve the model, GAMS retrieves information
saved in \t\a1. When finished solving
mymodel.gms, GAMS will save all information
including solutions in \t\a2 where it is ready to
be used later
Non-linear and Other Problem Forms
Set declarations
Parameter declarations
Data
Variable declarations
Equation declarations
Model
Specifying algebraic structure
Model specifications
SOLVE Ex USING LP MAXIMIZING Z;
Solve specifications SOLVE Ex USING MIP MAXIMIZING Z;
SOLVE Ex USING NLP MAXIMIZING Z;
Hands On 3
• Fix errors in Handson3error.gms and send an
electronic copy of the correct *.gms file. Hand in a
hard copy of *.lst file (from “Solution Report” to the
end).