Modifying Parameter in MCP ModelInstance

Questions on using the GAMS APIs (Python, .NET, etc.)
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Modifying Parameter in MCP ModelInstance

Postby Jan » 1 week ago

Dear all,

I try to modify a parameter of a ModelInstance representing and MCP model. A minimal example based on the MCP transport model from the library is given below. Instantiating the model, an error is raised from "gmoLoadDataLegacy" telling me that I have an unmatched column and no unmatched row (which reminds me on the unmatched variable/equation error of MCP models).

*** gmoLoadDataLegacy: fillMatches failed: matching error: nUnmatchedCols = 1 but nUnmatchedRows = 0
*** Could not load data from file: matching error: nUnmatchedCols = 1 but nUnmatchedRows = 0

Against the background, that the documentation states, that modifiers are converted to variables in the ModelInstance, I have two questions:

(1) Is it possible to manipulate MCP models using ModelInstances?
(2) If yes, can you provide me a hint what is going wrong with the code below?

Best regards

P.S. Versions: Python 2.7, Windows 7, GAMS 24.8.3

Code: Select all

# coding: utf-8
import gams

model_text = """$Title Transportation model as equilibrium problem (TRANSMCP,SEQ=126)
   Dantzig's original transportation model (TRNSPORT) is
   reformulated as a linear complementarity problem.  We first
   solve the model with fixed demand and supply quantities, and
   then we incorporate price-responsiveness on both sides of the

Dantzig, G B, Chapter 3.3. In Linear Programming and Extensions.
Princeton University Press, Princeton, New Jersey, 1963.


       i   canning plants   / seattle, san-diego /
       j   markets          / new-york, chicago, topeka / ;


       a(i)  capacity of plant i in cases (when prices are unity)
         /    seattle     350
              san-diego   600  /,

       b(j)  demand at market j in cases (when prices equal unity)
         /    new-york    325
              chicago     300
              topeka      275  /,

        esub(j)  price elasticity of demand (at prices equal to unity)
         /    new-york    1.5
              chicago     1.2
              topeka      2.0  /
        multdem    demand mulitplier   /1/ 

  Table d(i,j)  distance in thousands of miles
                    new-york       chicago      topeka
      seattle          2.5           1.7          1.8
      san-diego        2.5           1.8          1.4  ;

  Scalar f  freight in dollars per case per thousand miles  /90/ ;

  Parameter c(i,j)  transport cost in thousands of dollars per case ;

            c(i,j) = f * d(i,j) / 1000 ;

  Parameter pbar(j) reference price at demand node j;

  Positive variables
       w(i)             shadow price at supply node i,
       p(j)             shadow price at demand node j,
       x(i,j)           shipment quantities in cases;

       supply(i)        supply limit at plant i,
       fxdemand(j)      fixed demand at market j,
       prdemand(j)      price-responsive demand at market j,
       profit(i,j)      zero profit conditions;

profit(i,j)..   w(i) + c(i,j)   =g= p(j);

supply(i)..     a(i) =g= sum(j, x(i,j));

fxdemand(j)..   sum(i, x(i,j))  =g=  b(j)*multdem;

prdemand(j)..   sum(i, x(i,j))  =g=  b(j) * (pbar(j)/p(j))**esub(j);

*       declare models including specification of equation-variable
*       association:

  Model fixedqty / profit.x, supply.w, fxdemand.p/ ;
  Model equilqty / profit.x, supply.w, prdemand.p/ """

# initialize workspace, options, and checkpoint
ws = gams.GamsWorkspace(debug=2)
opt = ws.add_options()
opt.mcp = "PATH"
cp = ws.add_checkpoint()

# create checkpoint running GAMS file
j_modeltext = ws.add_job_from_string(model_text)

# intitialize GAMSModelInstance
mi = cp.add_modelinstance()

# add changing parameter to sync_db
multdem = mi.sync_db.add_parameter("multdem", 0, "uniform demand multiplier")

# instantiate the GAMSModelInstance and pass a model definition and GAMSModifier to declare bmult mutable
mi.instantiate("fixedqty using MCP", gams.GamsModifier(multdem), opt)

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