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Re: [Help-glpk] glpsol.exe differs from AMPL

From: glpk xypron
Subject: Re: [Help-glpk] glpsol.exe differs from AMPL
Date: Thu, 28 Jun 2012 05:34:44 +0200

Hello Michael,

AMPL is not a solver but an interface to a solver. Hence it is unclear which 
solver you are comparing GLPK to, might be IBM Ilog Cplex.

Both GLPK as well as IBM Ilog Cplex will provide **any** of the optimal MIP or 
LP solutions if successful. Hence the result is correct.

A model which has a lot of symmetries is very inefficient to solve.

You should try to add additional constraints that force the solution of your 
problem to be unique.

Best regards


-------- Original-Nachricht --------
> Datum: Wed, 27 Jun 2012 17:10:47 -0400
> Betreff: glpsol.exe differs from AMPL

> Hello:
> I am running glpk MIP and in AMPL and finding that the order of the
> solutions differ.  For example, say I have a model where I need to maximize an
> objective based upon paired values X, Y, and Z (and each pairing has a
> score).  I am not using any additional options for the solutions, eg 
> specifying
> branch&cut, etc.
> AMPL will tell me that the solution is X, Y, Z
> glpk will tell me the solution is Z, X, Y.
> I note that the sum of the scores for each solution is equivalent (as it
> should be as addition is commutative).
> The problem I am trying to reconcile is is there a physical difference
> between ordering objects X, then Y, then Z with the order of Z, then X, then
> Y?
> Not sure if this is clear to everyone, but the main point is I see
> differences.
> Michael Bramley

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