04 641 model inspection error

8/7/2019 04 641 Model Inspection Error

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Introduction to GAMS:Model inspection & Error messages

Dhazn Gillig&

Bruce A. McCarl

Department of Agricultural Economics

Texas A&M University


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Are you sure that you know EXACTLY what is in the model?

GAMS can cause one to lose touch with the exact optimization

model being solved, especially when one has a big complicated

data set containing many data calculations.

Thus, when setting up a model one may need to look at individual

equations and variables. GAMS permits this through the use of

model element displays using

! LIMROW and LIMCOL options

! GAMSCHK

Model Inspection


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LIMROW controls the number of rows listed in the equation listing

LIMCOL controls the number of columns listed in the variable listing

If we want more or less variables or equations we can do this by

altering the LIMROW and LIMCOL options

To eliminate => OPTION LIMROW = 0;OPTION LIMCOL = 0;

To expand => OPTION LIMROW = 100 ; (or any other number)

OPTION LIMCOL = 100 ;

Model Inspection Limrow / Limcol


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Min Cost of shipping goods from S to D

minimization of total cost across all possible

shipment routesLimited outgoing shipments from each S point i

Minimum demand constraint at each D point j

Nonnegative shipments

Limrow / Limcol Transportation problem illustration


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For equations we get something like



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Unfortunately, for large models the LIMROW/ LIMCOL option

can generate very substantial output files. An alternative way

of displaying models called GAMSCHK will be discussed in the

next class.

Model Inspection Limrow / Limcol


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Common Error Messages

What to do when ERRORS STRIKE and GAMS stops running?

Not to worry, when GAMS detects errors, it inserts coded error messages

inside the echo print in the *.LST file. All errors are marked with four

asterisks **** at the beginning of a line. One can use

to find where errors occur and fix them.


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Double click on Red Color Lines in the process window,

causes the cursor to jump to location in *.gms file where

error occurs.

Note: always concentrate on

fixing the first or first few errors

and ignore the others since

many subsequent errors may

result from a previous error.


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Double click on Black Color Lines in the process window,then a cursor will jump to a *.lst file where the error is listed.


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To have error messages displayed

immediately following an error line,

go to GAMS 20.7 directory and open

gmsprm95.txt (for window) or

gmsprmNT.txt (for NT), then typeerrmsg = 1

errmsg controls the location in the

listing file (*.lst) of error messages.

Alternatively,

errmsg = 0

error messages at the end of*.lst file

errmsg = 2

no error messages


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Note that in every equation all the subscripts (SETS) must behandled in every term. This means

! they either must be summed over, or

! they must appear in the equation definition


Not obeying this will cause errors as on next pages.


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Set is under control already.


Source set is used in conflicting ways. By appearing in the equation

specification (.. Command) SupplyBal(Source).. this defines these

equations for all elements of the set Source, but then how can you

sum over this. The answer is that you cannot and GAMS gives an error.Simultaneously, Destination set is not controlled. How do you FIX this?


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Uncontrolled set entered as constant


Here the set Source is not operated over in a sum or the ..

specification. So what is GAMS to do give an error message.

C E M


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Now for some other common errors

Floating entry ignored


We have made an error in typing of a table. We have not aligned the dataunder a column set name so GAMS does not know which data column a

number goes with and issues an error.


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Duplicate names

We already defined TransCost as a parameter and then we used it again in

equation specification. GAMS will object to these multiple declarations.



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Omitted set elements

We forgot to include San Diego element in Source set, but this San

Diego is used later in the PARAMETERS. Therefore, an error arises.



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Mismatched parentheses

We had an excess number of close ) in (A) but we had an excess number of

open ( in (B). We can use to check for parenthesis.

?

(A)

(B)



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Commo o essages

Entering improper nonlinear expressions

The CostSum equation contains nonlinear terms beyond the capacity of

the solver being used. In SOVLE statement, we define to solve this

problem as a LP problem.



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g

Using undefined data

We declared a TransCost as a parameter but we did not declare numerical

values to the TransCost and it is used in the CostSum equation.



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g

No variable, parameter, or equation definition

A CostSum equation is used but it has not been declared.



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g

Insufficient semi colon

We forgot to terminate the EQUATIONS statement with the semi colon.

;


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= or .. operator expected


The algebraic structure of equations must be specified by using .. notation.Here we omitted the .. in CostSum equation.



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Unknown symbol


We named a model Tranpor, but then later we solve the model called

Transport. Obviously, misspelled in the MODEL statement.

More Help on Error Messages


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More Help on Error Messages

Double click on a

topic to view details

Hands On


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Learning Objectives (handson4.gms):

1. Learn how to inspect a model structure using OPTION

LIMROW and LIMCOL

2. Learn how to find and fix errors

Things To Do

1. Please run handson2.gms and then fix errors and re-run the problem.

2. Please open handson3.gms and save it as handson4.gms. Alter yourobjective function from

to

Then run handson4.gms and inspect the model using OPTION

LIMROW and LIMCOL. Is there anything wrong with the model

structure or solutions?

References


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References

McCarl, B. A. Basic GAMS class.(http://ageco.tamu.edu/faculty/mccarl/mccarl.htm).

04 641 model inspection error

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