Originally posted by: SystemAdmin

[ppadegim said:]

Hi everyone.  I have just started using CPLEX and I don't quite understand how constraints with different dimensions interact.  I have a problem with a constraint as follows:  Y i,j,t + Z i,t - X i,t = 0.  The Y variable is defined as transfers from i to j in period t, with Z being demand left unsatisfied in location i at period t, and X being an initial deficit in location i, period t, which is defined in an earlier constraint.  My problem is that I do not understand the way that CPLEX reads i, j, and t and am therefore having trouble writing the input file so that the LP executes properly.  Hopefully this is clear enough for everyone to understand.  I appreciate any help that might be offered.

Thank you,

Paul
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Originally posted by: SystemAdmin

[prubin said:]

It would help to know how you are using CPLEX (interactive optimizer, C++ API, Java API, ...) and how you are trying to input the problem (.lp file, .mps file, type in from keyboard, read coefficients from a flat file into a C++/Java program, ...).

/Paul

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#DecisionOptimization

Originally posted by: SystemAdmin

[ppadegim said:]

I am using the interactive optimizer and inputting the problem as a text file.  The format is as follows:

Minimize Z i,t
subject to
c1: Y i,j,t + Z i,t - X i,t = 0
bounds
Y i,j,t >= 0
Z i,t >= 0
X i,t >= 0

Thank you very much,

Paul
#CPLEXOptimizers
#DecisionOptimization

Originally posted by: SystemAdmin

[prubin said:]

The interactive optimizer does not understand subscripting.  If you are typing the problem in, you need to substitute integers for subscripts as you type.  So you enter

c1: Y111 + Z11 - X11 = 0
c2: Y121 + Z11 - X11 = 0

etc.

(Incidentally, you might want to reconsider that particular set of constraints:  it forces Yi,j,t to equal Yi,k,t.)

Presumably you do not want to mess with all this yourself, so you might want to consider either (a) writing a program that expands your model and outputs it in either LP or MPS format, which you can then read into CPLEX or (b) use a modeling language (AMPL, GAMS, Mosel, OPL, MPL, ZIMPL, GMPL, ...) to convert your algebraic model to LP or MPS format and read it into CPLEX.  The first n-2 of those are commercial products, although I think there are free student versions (limited capacity) for some if not all of them.  ZIMPL and GMPL (part of GLPK) are open-source.  I think both implement a subset of the AMPL language.

/Paul
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