Power of GAMS - Department of Agricultural...

Outline Features Library Tips

Power of GAMS

Pei Huang1

1Department of Agricultural Economics

Texas A&M University

Based on materials written by Gillig & McCarl and improved upon by many previous lab instructors

Special thanks to Mario Andres Fernandez

Pei Huang | Texas A&M University | AGEC 641 Lab Session, Fall 2013 1 / 17

Outline

1 Why should we use GAMS?

2 GAMS Features

3 Model Library

4 Operating Tips

Why should we use GAMS?

Algebraic modeling

Context changesExpandability - Expanding scopeExpandability - Augmenting existing models

Self-documenting nature

Small to large modeling

GAMS solvers

Model library

Use by others

Algebraic modeling

GAMS solvers

Model library

Use by others

Algebraic modeling

GAMS solvers

Model library

Use by others

Algebraic modeling

GAMS solvers

Model library

Use by others

Algebraic modeling

GAMS solvers

Model library

Use by others

Algebraic modeling

GAMS solvers

Model library

Use by others

Algebraic modeling

GAMS solvers

Model library

Use by others

Context Changes

ExampleSETSProcess 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

/MakeChair Chair production

MakeTable Table production

MakeLamp Lamp production

/Cap Max Production Capacity

Context changes but the model structure is the SAME! Only elementsin sets are changed.

Context Changes

ExampleTABLE ResourceUse(Resource,Process)

Resource usedCorn Wheat Cotton

Land 1 1 1Labor 6 4 8 ;

PARAMETERRevenue(Process)

Revenues from process production

/Corn 109Wheat 90Cotton 115 /

ResourceAvail(Resource)

Resource availability

/Land 100Labor 500 /;

Resource usedMakeChair MakeTable MakeLamp

Cap 2 3 1.1Labor 1 2 0.5 ;

/MakeChair 11MakeTable 10MakeLamp 12 /ResourceAvail(Resource)

/Cap 12Labor 5/;

Context changes but the model structure is the SAME! The only partmodi�ed is the data.

Context Changes

Land 1 1 1Labor 6 4 8 ;

Cap 2 3 1.1Labor 1 2 0.5 ;

/Cap 12Labor 5/;

Context Changes

Land 1 1 1Labor 6 4 8 ;

Cap 2 3 1.1Labor 1 2 0.5 ;

/Cap 12Labor 5/;

Comparison

Expandability - Expanding Scope

ExampleSETSProcess Production process/Corn Corn productionWheat Wheat productionCotton Cotton productionSoybean Soybeans production /

Resource Resource item used/Land Land used by processLabor Labor used by process/;

TABLE ResourceUse(Resource,Process) Resource usedCorn Wheat Cotton Soybean

Land 1 1 1 1Labor 6 4 8 4 ;

PARAMETERRevenue(Process) Revenues from process production/Corn 109Wheat 90Cotton 115Soybean 95 /

ResourceAvail(Resource) Resource availability/Land 100Labor 500 /;

Instead of growing 3crops, now a farmer alsowants to grow soybeans.One needs only to modifyan element in Processset, ResourceUse table,and Revenue parameter.

Other data and themodel structure remainsthe SAME!

Expandability - Augment Existing Models

ExamplePARAMETERRevenue(Process) Revenues from process production/Corn 109Wheat 90Cotton 115Soybean 95 /

ResourceAvail(Resource) Resource availability/Land 100Labor 500 /

MinLand(Process) Minimum land requirement/Corn 0Wheat 10Cotton 0Soybean 0 /;

EQUATIONSObjective Maximize farm incomeResourceEq(Resource) Resource constraintMinLandReq(Process) Minimum land requirement ;

MinlandReq(Process)..ResourceUse(“Land”,Process)*Production(Process)=g= MinLand(Process) ;

Modi�cation includesadding data on minimumland use, and equationspeci�cation onminimum land use.

Other data and themodel structure remainsthe SAME!

Self-documenting Nature

GAMS allows one to add explanatory text when naming SETS,PARAMETERS, TABLES, VARIABLES, EQUATIONS, but it is agood habit to name them with easy-understanding words instead ofsimple letters.

Which form between the examples below clearly shows the context ofitself?

GAMS allows one to add explanatory text when naming SETS,PARAMETERS, TABLES, VARIABLES, EQUATIONS, but it is agood habit to name them with easy-understanding words instead ofsimple letters.

Which form between the examples below clearly shows the context ofitself?

Always remember commenting your code. Comments can not onlyhelp others who read your code, but also help yourself for futurereferences.

Comment a line: asterisk �*�

Comment multiple lines: put comments between $ONTEXT and$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. SETSwith many items.

Using a small data set allows up to examine the model structure andfunction easier and better. Then later one can use the same algebrafor a large problem.

Use subsetsUse small group of scenarios (1 or 2)Use aggregate regions

GAMS Solvers

GAMS integrates many solvers that can solve most optimizationproblems.

Specify the solver in the SOLVE statement

Example

SOLVE MyProblem using LP maximizing Z ;

SOLVE MyProblem using MIP maximizing Z ;

SOLVE MyProblem using NLP maximizing Z ;

GAMS Solvers

GAMS integrates many solvers that can solve most optimizationproblems.

Specify the solver in the SOLVE statement

Example

SOLVE MyProblem using LP maximizing Z ;

SOLVE MyProblem using MIP maximizing Z ;

SOLVE MyProblem using NLP maximizing Z ;

Model Library

GAMS has been used as a standard in optimization models in many�elds.

Models exist from experienced users that address similar problems

Textbook

Fixing Models Book

GAMS Library

GAMS Newsletter

Model Library in GAMS

Models are available through the GAMS library which is directlyincluded in the IDE.

Use by Others

Two �les: data.gms andmymodel.gms.

Create a �t� folder (or any othernames) under the projectdirectory.

Type the content in the red boxbefore running data.gms.

When �nished running data.gmswhich includes all of data,GAMS will save all theinformation in \t\a1 where it isready to be used.

Use by Others

To solve the model,GAMS retrievesinformation on data thatwas saved in \t\a1.

Type the content in thered box before runningmymodel.gms.

When �nished solvingmymodel.gms, GAMS willsave all informationincluding solutions in\t\a2 where it is ready tobe used later, say, reportwriting.

Use by Others

Appendix

Heads On 3

Fix errors in Handson3error.gms and send an electronic copy of thecorrect *.gms �le via email.

Hand in a hard copy of *.lst �le (from �Solution Report� to the end).

Power of GAMS - Department of Agricultural...

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